The Manufacture and Properties of Killed Bessemer Steel

Transcription

1 AMERICAN,INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No. 169a. -- (CLA~ C. IRON AND STEEL DNISION. No. 3~7) DISCUSSION OF THIS P'APER ISINVITED. Discussion in writing (a co ies) may be sent to the Secretary. American Institute of Minin and Metallurgical Engineers. a9 West 39tg Street. New.York 18. N. Y. Unless special arran ement is mad, discussion of this paper will close July Any discussion offered thereafter should pre7erably be in the form of a new paper. The Manufacture and Properties of Killed Bessemer Steel BY E. - C. WRIGHT,* MEMBEB A.I.M.E. (New York Meeting. February.1944) THE bessemer process is nearly one of the total in 1900 to 37.1 per cent in 1910, hundred years old. William Kelly, the and only 6.8 per cent of the total in American inventor, was able to demon- The metallurgical concepts existent bestrate that he had accomplished the pneu- tween 1910 and 1920 caused the writing of matic purification of molten pig iron as many specifications of very low phosphorus early as 1847; Henry Bessemer's English and sulphur content, which definitely patent was granted in The applica- barred bessqner steel from many comtion of this steelmaking process since then modities. Now sulphur is being added to and the present position of bessemer steel many open-hearth and bessemer steels in are concisely discussed on pages 355 and amounts far exceeding the maximum sul- 356 of the fifth edition of "The Making, phur content of good bessemer steel. Re- Shaping and Treating of Steel." phosphorizing is also widely practiced for The great expansion of the automotive, several specific applications. Even today, machine-tool, chemical and oil industries the chemical-analysis limits of many imporafter 1910 opened up a large market for tant steel specifications have an'arbitrary special steels, whereas the output of steel and archaic tinge: The old standard "0.30 before that time was mainly for structhal to 0.60 per cent manganese" might be purposes. The need for building more blast- mentioned in this connection as similar to furnace capacity complementary to bes- the rigid requirements for maximum sulsemer operations, combined with the phur and phosphorus that were established availability of the necessary amounts of.at least 25 years ago. steel scrap, both contributed to the exten- The literature is full of vague andcomsion of open-hearth plants at the expense plex discussions of the effects of nitrogen of besiemer capacity. The ability of the and oxygen where these elements existed open-hearth furnaces to consume virtually in steel in amounts less.than 0.02 per cent. all types of raw materials as far as phos- Many failures have been attributed to phorui content is concerned, and the need these elements without reasonable proof for selecting low-phosphorus ores of rapidly on which to base the conclusion. Methods decreasing sources for the bessemer, also of determining oxygen in steel require such played a part in the reduction of bessemer complicated equipment that only a few steelmaking operations and the increase in research. laboratories have the required basic open-hearth plants. As a consequence, apparatus. Even so, the accuracy of oxygen the proportion'.of bessemer steel made in determinations in steel has been only.rethis country decreased from 65.7 per cent cently reproducible. Up to the present time, -- no one has succeeded in obtaining a sample ~~~~~~~i~~ received at the ofice of the of molten steel that accurately indicates ~nstitute N~V. IS the full oxygen content of the metal in the Assistant to President. National Tube Co.. Pittsburgh. - Pennsylvania. furnace or ladle because the molten sample Copyright by the American Institute of Mining and Metallurgical Engineers. Inc, METALS TECHNOLOGY. June Printed in U. S. A.

2 2 THE MANUFACTURE AND PROPERTIES OF KILLED BESSEMER STEEL takes up oxygen from the air in pouring. Past discussions of the oxygen content of steel have little quantitative foundation. In the case of nitrogen it has lately been learned that many high-grade electricfurnace steels have considerable amounts of this element although generally lower than the per cent nitrogen that is normal for bessemer steel. It has also lately become the vogue to add nitrogen to some open-hearth heats (up to 0.02 per cent), as these additions have been useful in regulating heat-treatment and grain-size control. Bessemer steel has been condemned frequently, and one may say erroneously, because of its higher nitrogen and phosphorus content. In this present emergency, the time has arrived for a thorough survey of the characteristics of bessemer steel, especially in view of the shortage of high-grade scrap and the enormous demand for all kinds of steel. The gradual decrease in the manufacture of bessemer steel over the years can be traced to economic and technical factors. Neglecting the economic phases, which vary widely throughout the years, it is believed that the properties of the bessemer steel made in the United States have changed very little during the past 40 years. There have been no new installations of bessemer-steel plants since the last war except where bessemer equipment was installed in conjunction with duplex open-hearth melting. As a consequence, most of the bessemer-steel plants in the United States are obsolete, with small converting vessels and antiquated handling, mixing and teeming equipment. The fundamental methods of making bessemer steel have changed little, in spite of the great amount of work that has been done to develop instruments to control the blowing of the molten iron. During the same period very important improvements have been made in the construction and in operating practices on both basic openhearth and electric melting furnaces. After the first World War, the greatly increased demand for special steels-for gears, bearings, and other important automotive parts, for instance-caused a rapid expansion in the use of special alloy steels and a considerable increase in the amount of electric-furnace melting capacity, since it was found that open-hearth furnaces were not generally suited for the consistent manufacture of certain highcualitv specialties. A trend to re~lace open-hearth steel with electric-furnace steel set in, as previously open-hearth steel displaced bessemer steel. This trend continues, as indicated by the enormous expansion in electric-furnace capacity during the present emergency. About 1925 this situation led to a serious consideration of the fundamental physical chemical principles involved in the making of steel, particularly as related to basic open-hearth melting practice. During the past 20 years much valuable work has been done on such important features as slag control, oxidation of the open-hearth bath, and particularly the deoxidation of the open-hearth steel both in the furnace and in the ladle. Very illuminating information as to the effect of deoxidizing agents upon such vital properties of the steel as the hardenability, carburizing characteristics, properties at low temperatures, welding properties, grain size, was obtained in these investigations. The most important principle encountered in these studies was that a control of the oxygen in the steel-melting bath and in the finished steel had a profound influence upon most of the important properties of the finished heat. This work enabled the production in basic open-hearth furnaces of an increased volume of high-grade carbon steels, and also alloy steels that could be consistently produced with definite grain-size control, hardenability, and so forth. Coincident with this im-

3 E. C. WRIGHT 3 provement, the steels also exhibited more uniform forging characteristics, better yield and improved surface. It is the writer's opinion that much of the openhearth steel made before 1920 was in many respects as lacking in uniformity as the bessemer steel produced before that date. In the National Tube Co. the improvement in quality of the open-hearth steel has caused an increased yield of product of at least 20 per cent in the past 15 years. The majority of the high-quality steels now made in the open hearth are fully killed steels treated with definite amounts of the proper deoxidizing agents such as silicon, aluminum, titanium and vanadium. Most of the bessemer steel made in the United States has been of the undeoxidized or "open" type. These steels are usually termed rimmed, capped or semikilled. They are high in oxygen, freeze in the ingot mold with highly segregated center sections, have surface blowholes, and are extremely erratic in hardenability, grain size and susceptibility to strain-aging and low-temperature brittleness. Such steels generally are not suitable for forging, cold-forming or heat-treating operations. The improvements in open-hearth furnace melting and control discussed above were a vital feature in the improvement of making, seamless tubes. This difficult forging dperation requires a billet of uniform density and forging characteristics. Satisfactory seamless tubes have never been produced with uniform practice from open-hearth steels of the rimmed or capped ingot type in the National Tube Co. The physical chemical principles investigated in connection with open-hearth steel melting apply equally well to the processes involved in making bessemer steel. The only difference between the two methods is the rate at which the reactions occur. In the bessemer process the hot metal is oxidized by the air blown through the bath, and thereby removing practically all the silicon and manganese, and at the time of the drop of the carbon flame above the converter, the carbon has reached the level of about 0.04 per cent. In the openhearth furnace the oxidation of the carbon, silicon and manganese of the bath is accomplished slowly by means of the oxygen from scale on the melting scrap, the oxygen in the slag, and the iron ore added as charge ore or feed ore to the bath. The elimination of the silicon, manganese and carbon proceeds in a way similar to the reaction in the bessemer but at a much slower rate. This enables the melter to control the rate of elimination of carbon and to arrest the carbon drop at any desired level. However, in making a heat of low-carbon steel in the open-hearth furnace, where a tap carbon of 0.10 per cent may be the aim, the characteristics of such a bath with respect to the silicon, manganese and oxygen approach a point of chemical equilibrium similar to that existing in the blown metal at the end of the blow in the bessemer converter. The very low-carbon open-hearth bath may be treated with additions of ferrosilicon or spiegel in the furnace to partially deoxidize the FeO, and the molten metal may then be given further additions of silicon and aluminum in the ladle to thoroughly kill the heat. The manufacture of a forginggrade heat of low-carbon open-hearth steel generally follows this procedure. Realizing that the physical chemical principles underlying the bessemer and open-hearth processes must be the same, the metallurgist naturally asked why bessemer steel could not be thoroughly deoxidized and finished in the same way as open-hearth steel. Following this line of reasoning, heats of blown besserner metal were tapped into teeming ladles and treated with ferromanganese, ferrosilicon, aluminum and other deoxidizers much as openhearth heats are treated. The results.of such practice were in many ways inconsistent and a thoroughly killed steel of uniform characteristics was not produced

4 4 THE MANUFACTURE AND PROPERTIES OF KILLED BESSEMER STEEL successfully in this manner. This was silicon, carbon and manganese as reprebelieved to be because either the blown senting the general agreement on these bessemer-converter metal was overoxidized values at the present time. The curves are or the mixing of the deoxidizing reagents based on calculations from Chipman's data. 1 FIG. I.--EQUILIBRIUM BETWEEN OXYGEN AND OTHER ELEMENTS IN LIQUID STEEL AT 1600OC. [FeOl = 4.49 [OI in the teeming ladle was not sufficiently uniform to develop a thorough deoxidation of the metal. Although theoretically 2 lb. of aluminum per ton should thoroughly deoxidize blown metal with 0.03 per cent carbon, additions of as much as 6 lb. of aluminum per ton would not effectively kill the heats in the ladle. The splendid paper by John Chipman1 on the physical chemistry of steel at 16oo~C. (2910 F.) has done much to clarify this subject. The work of Larsen, Herty and others also has contributed greatly to an understanding of the equilibrium between oxygen in molten steel and such deoxidizing agents as carbon, manganese, silicon, aluminum, vanadium. Figs. I and 2 show the equilibrium proportions of oxygen in molten steel at 16m C. (2910 F.) and 17m C. (3060 F.) with aluminum, 1 References are at the end of the paper. The influence of temperature on these equilibria is extremely important. For example, carbon is a much more powerful deoxidizer than silicon or manganese at temperatures above 16m C. (2910 F.) when the carbon content of the bath exceeds 0.10 per cent. The temperature of bessemer steel in the vessel generally exceeds 1600 C. (2910 F.). Recognizing this principle, attempts were made to deoxidize blown bessemer metal with additions of carbon and eliminate the oxygen in the bath as carbon gases instead of producing solid precipitated oxides such as SiOz and AlzOs, which remain to a considerable extent as inclusions in the steel bath. The simplest and most thorough method of treating a blown converter bath with carbon is to add hot metal, which contains approximately 4 per cent carbon, as well as some manganese and silicon. The action

5 E. C. WRIGHT 5 of such additions to fully blown bessemer heats gave a very thorough preliminary deoxidation, much like the "blocking" of an open-hearth bath by addition of metal blown to 0.04 per cent carbon are treated with 1400 lb. of hot metal, added in each vessel after blowing, and then are poured into a so-ton ladle. If no reac- ferrosilicon or spiegel. To bessemer blows were added the proper amounts of hot metal as converter additions. The steel was then poured into a teeming ladle and treated with the required amounts of ferromanganese, ferrosilicon and aluminum to deoxidize the blocked heat, as in openhearth practice. The bessemer heats so treated were found to be thoroughly killed and of good forging quality. They could be teemed into regular hot-top ingot molds, rolled into tube rounds and satisfactory seamless tubes with practices equaling results obtained on open-hearth steels. A specific example of the methods* of "killing" or thoroughly deoxidizing a bessemer heat is offered by way of illustration. Two 2s-ton blows of bessemer The manufacture and application of the bessemer steels described are covered by United States Patent and pending applications. tion occurred, the carbon content of this mixture would approximate 0.16 per cent. The mixing of the blown metal and hot metal in the vessel is almost instantaneous and the elimination from the metal of carbon and oxygen in the form of CO gas is extremely rapid. In fact, a large puff of flame over the mouth of the converter appears just after addition of the molten iron. In 100,000 lb. of blown metal containing 0.08 per cent oxygen, the amount of oxygen is approximately 80 lb.; 2800 lb. of hot metal with 4.25 per cent carbon and 1.5 per cent silicon contains 119 lb. of carbon and 42 lb. of silicon. The 80 lb. of oxygen should require 60 lb. of carbon for deoxidation. It is assumed that the finished steel contains per cent oxygen, as thoroughly killed heats show this amount in several analyses. The oxygen removed is thus 65 lb., requiring 48 lb. of carbon. The

6 6 THE MANUFACTURE AND PROPERTIES OF KILLED BESSEMER STEEL TABLE I.-Chemical Balance in a Killed Bessemer Heat Constituent Efficiency, Carbon Silicon Manganese Oxygen 0.36 per cent FeO I 0.0r lb. of carbon in the 2800 lb. of hot mdql ic partly ~liminatrrl ac r0 ~c and the carbon content of the resultant steel increased from approximately 0.04 to 0.12 per cent, indicating a carbon-oxygen equilibrium at 0.12 per cent carbon. This increase in carbon content of 0.08 per cent in ~m,ooo lb. of blown metal requires 80 lb. of carbon. Accordingly the 40 lb. of carbon has reacted with 53 lb. of oxygen. This result demonstrates that most of the oxygen in the blown metal is eliminated as CO. After this reaction, the steel may be treated in the usual manner with the proper additions of ferrosilicon, I to 3 lb. of aluminum per ton of steel, and sufficient ferromanganese for the final manganese content desired in the steel. The efficiency of the manganese and silicon additions to the ladle is as high as in open-hearth heats tapped at the 0.12 per cent carbon level. Table I shows the chemical balance between the additions in the example given. A further indication of the effectiveness of the carbon deoxidation is found in inclusion ratings of thoroughly killed bessemer heats. The standard method of rating inclusions by means of microscopic examination of nine sample cuts taken from the top, middle and bottom of the first, middle and last ingot of several thoroughly killed bessemer heats reveals a cleanliness superior to that of open-hearth heats of the same carbon content, treated with the same deoxidizing reagents. The carbon content of the steel may be raised to any level desired by increasing the amount of hot metal used; satisfactory heats with rarhon as high as 0.~5 per cent and over I per cent manganese have been made. The addition of hot metal to openhearth heats, instead of ferrosilicon or spiegel in the furnace, has also been successful. The higher temperature of the bessemer steel as compared with the usual temperature for open-hearth steel apparently makes the molten iron more effective as a deoxidizer in the bessemer heats. It was soon found that the thoroughly deoxidized bessemer heats melted with this practice were fundamentally different from undeoxidized bessemer steel. Up to the present time over 300,000 tons (between 6,000 and 10,000 heats) of thoroughly deoxidized bessemer steel have been made with this procedure and converted into almost every size of seamless pipe. During the past five years the large tonnage of killed bessemer steel so made has been investigated to determine its properties in comparison with the old standard rimmed or capped bessemer steel, as well as with killed open-hearth steel. Many data concerning most of the important physical properties of steel have been obtained. The results of these tests are presented in order that the reader may compare and realize the fundamental difference exhibited by the thoroughly deoxidized bessemer steel in contrast with the erratic properties of bessemer steel formerly produced without this type of deoxidation. 'rr

7 E. C. WRIGHT 7 The majority of bessemer steels made with this new practice for applications in seamless pipe have been produced to ' analyses aiming at 0.12,to 0.20 per cent The striking features of these tests are the high yield strength and tensile strength for the low carbon content and the high ratio of yield strength to tensile strength. BASIC OPEN HEARTH STEEL 95,000 MINIMUM TENSILE ZEO ACID BESSEMER STEEL,000 MINIMUM TENSILE 70,OCO MINIMUMTENBILE FIG. 3.-RESISTANCE D4 TO EXTERNAL PRESSURE OF PIPE FOR SEVERAL GRADES OF STEEL. carbon, 0.30 to 0.60 per cent manganese, In addition, determination of elastic 0.07 to 0.11 per cent phosphorus, 0.05 per limit by the plotting of stress-strain diacent maximum sulphur, and 0.15 to 0.30 grams has shown that steel of this comper cent silicon. Tensile tests have been position has a uniformly high elastic limit, taken on every heat produced to this generally exceeding 95 per cent of the 0.2 analysis. The average and minimum tensile per cent set yield strength. The ratio of the properties for some 1800 tensile tests are yield strength to the tensile strength has as follows: consistently been above 70 per cent. These two features of high elastic limit and high Aver- Mini- ratio of yield strength to tensile strength age mum make these steels distinctly superior to Yield strength, 0.01 per cent set, lb. per sq. in Yield strength, 0.2 per cent set, lb. per sq. in.: Tensile strength, lb. per sq. in.. Elongation in 2 in., per cent ,020 43,400 55,000 75, ,wo 66, open-hearth steel of the same tensile strength, as the latter generally exhibits a ratio of yield strength to tensile strength of about 62 per cent and a ratio of elastic limit to yield strength that may vary from 60 to go per cent. This is of particular advantage in pipe used for structural pur-

8 8 THE MANUFACTURE AND PROPERTIES OF IULLED BESSEMER STEEL poses, such as tubular piling, drive pipe, and products subject to high pressures, such as line pipe. The greater stiffness resulting from the higher yield-strength ratio siderably superior to the standard openhearth steel casing of 70,000 lb. per sq. in. tensile strength, known as H-40 grade. Fig. 3 shows the collapse resistance of has made this type of pipe extremely satisfactory for drive pipe, which is pounded into the ground much like piling, since it exhibits much less crushing and deformation in driving the sections, permitting the use of harder blows and faster driving. Extensive investigations of the collapse resistance of pipe indicate clearly that the elastic limit or proportional limit of the pipe steel is of prime importance in obtaining higher collapse resistance to external pressure. It is believed that the small content of phosphorus in this deoxidized bessemer steel is the main factor contributing to the high elastic ratios and high proportional limits. Collapse tests made on many sizes of pipe show that the collapse resistance of the low-carbon bessemer-steel casing is equivalent to the collapse resistance of open-hearth alloy-steel casing, designated as A.P.1. grade J-55, and con- various sizes of casing for the H-40 and 5-55 A.P.I. open-hearth grades compared with that of the killed bessemer-steel grade. The A.P.I seamless grade is made from open-hearth steel with minimum yield strength of 55,000 lb. per sq. in. and minimum tensile strength of 95,000 lb. per sq. in. The ordinary analysis of this openhearth steel for 5-55 casing is of the order of 0.35 to 0.45 per cent carbon, 1.00 to 1.30 per cent manganese. The fatigue properties of the killed bessemer steel are somewhat unusual in that the tests show a high endurance limit considerably above 50 per cent of the tensile strength of the material. Similar fatigue tests on open-hearth steels in the normalized state usually show endurance limits ranging from 45 to 50 per cent of the tensile strength. Fig. 4 shows fatigue tests on two types of open-hearth steel for

9 E. C. WIUGHT 9 comparison with the endurance limit of the with a carbon content of 0.12 to 0.40 per deoxidized bessemer steel. The superiority cent, show that ductility and toughness of of the killed bessemer steel in this respect this material are equal to open-hearth is quite evident. steel of similar hardness and tensile As stated previously, the bessemer steel described contains approximately per cent nitrogen and from 0.06 to 0.1 I per cent phosphorus. The sulphur is generally as low as in open-hearth steel, ranging from to per cent. Although many statements in the literature infer that both phosphorus and nitrogen tend to cause brittleness and lack of ductility in steel, thousands of tests on the thoroughly killed bessemer steel, strength in spite of the presence of appreciable amounts of nitrogen and phosphorus. The ductility of the killed bessemer steel as measured by percentage of elongation and reduction of area in tensile tests is normal in every respect. As shown in the average physical properties reported, the elongation in 2 in. is 35 per cent for an average tensile strength of 75,000 Ib. per sq. in. The elongation on basic open-hearth steels of this tensile strength is in no way

10 I0 THE MANUFACTURE AND PROPER TIES OF KILLED BESSEMER STEEL superior. The reduction of area as determined on round tensile tests is well over 60 per cent for the thoroughly killed acid bessemer steel. Ductility determinations based on flattening tests on pipe and on bending tests also demonstrate that killed bessemer steel is in every way equal to basic open-hearth steel of the same hardness. In the early stages of the manufacture of the bessemer seamless pipe, every piece of material was subjected to flattening tests, as it was suspected that brittle specimens might be encounrere6. sever-; hundred thousand such flattening tests were made without revealing brittle failures and, moreover, the pipe made from the killed bessemer steel flattened to the same degree as open-hearth steel of equivalent hardness. The toughness of the killed bessemer steel has been thoroughly explored by impact tests at both room temperature and minus 2s F on every tenth heat produced. Up to the present time more than 5000 Charpy impact tests have been so made. The majority of these tests have been cut from pipe sections with a pipewall thickness less than the in. width of the Charpy test piece. In many cases so-called half-size or two-thirds size Charpy impact tests have been necessary. The depth through the notch of the impact test was the same as that for the standard Charpy impact specimen. Fig. 5A shows the results of Charpy impact tests in both the hot-rolled state and normalized state of heats tested at 7s F., and Fig. 5B shows the results at minus 25OF. Tests at 32OF. and o F. fall between the limits exhibited by the two temperatures in the figures. The results are plotted as frequency distributions of several thousand tests at each temperature. It should be emphasized that the majority of the Charpy impact tests were made on Charpy specimens two thirds the ordinary width and the impact value would be approximately 30 per cent higher if made on full-width Charpy test pieces. The frequency distribution is similar to results obtained with finegrained open-hearth steels of approximately 75,000 lb. per sq. in. tensile strength. These results clearly show that the fully killed bessemer steel is tough at temperatures at least as low as minus 25OF. A few tests made at minus 50 F have also exhibited similar properties. The excellent ductility and impact toughness exhibited by this bessemer steel, with ~allvll iviit~iit!x>;-czz 2-12 :I?'! r~, A n npr cent, show very definitely that phosphorus and nitrogen, even when present in considerable amounts, do not have a detrimental effect on these properties. It is concluded that the form in which the oxygen exists in the steel is probably the predominating factor in controlling toughness at subzero temperatures. It is well known that open-hearth steels in the semikilled or coarse-grained state show very poor impact properties below 32OF. The material used at temperatures between minus 25' and minus 50 F., whether openhearth or bessemer, must be fine-grained material, thoroughly treated with aluminum additions greater than 156 lb. per ton of ingots. Studies of the welding qualities of steels during recent years have emphasized that elements that impart high hardenability to the steel, such as carbon, manganese, chromium, and molybdenum, make welding more difficult as the percentage of these elements is increased in the steel. It has been found that standard hardenability tests similar to the Jominy endquench test form an important index of the ease with which steel may be welded and also the degree of preheating necessary for safe welding procedure. An open-hearth steel of 75,000 lb. per sq. in. tensile strength must contain at least 0.30 per cent carbon * and I per cent manganese. At the present time this represents about the optimum content of these two constituents that. x I

11 E. C. WRIGHT I I may be safely welded without resorting to described herein contains less than 0.25 per costly preheating and postheating operations. Even so, the Jominy hardenability cent carbon and less than 0.60 per cent manganese. The hardenability of the of such an open-hearth steel is fairly high, material adjacent to the weld.is con- COOLING RATE, deg. f. per second at 1300 deg. as the hardness adjacent to welds in openhearth steel of 75,000 lb. per sq. in. tensile strength is apt to approach 45 Rockwell C. This may be readily determined by means of spot bead tests wherein the steel to be welded receives a spot deposit from the welding rod and is then' sectioned and etched for hardness determinations. Such tests made on open-hearth steel of 75,000 Ib. per sq. in. tensile strength have shown Rockwell hardness up to 45 on the C scale, whereas similar tests on the bessemer steel 'of 75,000 lb. per sq. in. tensile strength seldom develop Rockwell hardness exceeding 27 on the C scale. The bessemer steel of 75,000 lb. per sq. in. tensile strength FIG. 6.-END-QUENCH TEST. siderably less than with open-hearth steels of the same tensile strength. This has been further verified through standard Jominy end-quench hardenability tests, and the comparison between the open-hearth and killed bessemer steel is shown in Fig. 6. The open-hearth steel shown has a lower tensile strength than thk bessemer steel, and, even though it represents fine-grained quality, its hardenability is much greater than that of the killed bessemer steel. These results lead to the conclusion that for high-strength structures involving welding, the killed bessemer steel is superior to the grades of open-hearth,

12 I2 THE MANUFACTURE AND PROPERTIES OF KILLED BESSEMER STEEL steels of similar tensile strength that are now available to the trade. The properties of the killed bessemer steel at elevated temperatures have been bessemer grades are strongly susceptible to increased strength and loss of ductility in the so-called blue-heat range between 400' and 6o0 F. In order to determine how c $100 0 g Y 3 70 B so 2? so U ; u B ; lo z 02 0 W REOUCTION DF AREA AND ELONGATION TEMPERATURE DEE. FAHR.- HUNOREOS -COMPARISON OF TENSILE PROPERTIES AT ELEVATED OPEN-HEARTH AND BESSEMER CAaBON STEEL (0.17 PER CENT C). investigated at the United States Steel Corporation Research Laboratory by means of short-time tensile tests at elevated temperatures as well as long-time creep tests at 850' and IOOOOF. In general, short-time high-temperature tests of killed bessemer steel and killed open-hearth steel, both in the fine-grained state, are similar with respect to tensile strength, yield strength, elongation, and reduction of area. The results of tests made on both types of steel are shown in Fig. 7. It has long been known that undeoxidized steels of both open-hearth and TEMPERATURE OF NORMALIZED the thoroughly killed bessemer steel reacted in this temperature region in comparison with other steels, short-time hightemperature tensile tests have been made on the following types of steels: Undeoxidied capped bessemer steel Undeoxidized capped open-hearth steel Silicon-killed coarse-grained open-hearth steel Silicon-aluminum-killed fine-grained openhearth steel Siliwnaluminum-killed fine-grained bessemer steel Fig. 8 exhibits the results of these tests and shows that all the undeoxidized steels

13 E. C. WRIGHT I3 I are very susceptible to blue-heat brittleness of the fine-grained open-hearth steel in this temperature range, whereas both approximated 6400 lb. per sq. in. at 850 F. the fine-grained open-hearth and bessemer compared with 10,200 lb. per sq. in. for steels are practically free from this char- the killed bessemer steel with correspond- L b ~ '! I I I500 I I I i! I r I I 600 I I u! I I TEMPERATURE OF: FIG. 8.-TENSILE STRENGTH OF STEELS AT BLUE-HEAT TEMPERATURES. acteristic. It is therefore concluded that the fine-grained thoroughly killed bessemer steel, when made according to the practices given, is apt to be less susceptible to strain-hardening and effects of blue-heat brittleness than many steels used in the past. Long-time creep tests were conducted at 850' and ~oooof. on fine-grained openhearth steel and he-grained siliconaluminum-killed bessemer steel. Results of these tests show a dehite superiority for the bessemer steel (Fig. 9). The creep limit ing values at IOOOOF. of I 500 lb. per sq. in. for the fine-grained open-hearth as against 3300 lb. per sq. in. for the bessemer steel. It is believed that the superior creep resistance of the killed bessemer steel is largely due to its phosphorus content, as has been noted by numerous previous investigators, particularly Gillett,s Crossa and their co-workers. It is well known that molybdenum increases the creep resistance of steels, and in order to determine whether phosphorus would supplement the effect of molybdenum, creep tests were made on

14 I4 THE MANUFACTURE AND PROPERTIES OF KILLED BESSEUER STEEL steels containingo.5oper cent molybrlenum and 0.10 per cent phosphorus. Here again the influence of phosphorus was verified; a low-phosphorus steel (0.02 per cent) con- It has long been considered that ordinary bessemer steels, which in the past have been of the undeoxidized type, are extremely susceptible to work-hardening and taining 0.50 per cent molybdenum exhibits a creep limit of 9,000 to 10,ooo lb. per sq. in. at IOOOOF. whereas a steel containing 0.10 per cent phosphorus and 0.50 per cent molybdenum showed a creep limit at ~ooo"f. of 13,wo Ib. per sq. in. These results should indicate that bessemer steels treated with molybdenum would be considerably superior in high-temperature strength to molybdenum-bearing openhearth steels used in the past, or openhearth heats could be rephosphorized to level of 0.10 per cent in order to increase the high-temperature strength. very sensitive to strain-aging. These properties can be readily determined by means of the strain-sensitivity test developed by Work4 of Jones and Laughlin Steel Corporation. This test consists in the drawing of a bar, previously forged, normalized, and machined with a tapered outside diameter, through a die in order to obtain a reduction in area along the bar length varying from o to 10 per cent. The bars after cold-drawing are tested in the as-strained state, while other bars are heated to temperatures in the region of 400" to 550 F. in order to develop a

15 E. C. WRIGHT Is.strain-aged condition. The characteristics of the fine-grained killed bessemer:steel were compared with those of several other types of carbon steel by means of this viously. For both the open-hearth and bessemer steels, only the fine-grained fully deoxidized steels are free from strainaging and blue-heat brittleness, and tough C 0 2 so s - - N s40 a 20 z 3 COARSE GRAIN D.H strain-sensitivity test. The results are shown in Fig. 10 for six different steels. The undeoxidized and partially deoxidized coarse-grained steels show a high strain sensitivity, while the fine-grained siliconaluminum-killed open-hearth and bessemer steels are quite resistant to strain-aging effects. These results coordinate closely with the high-temperature tensile tests and low-temperature impact tests given pre- at low temperatures. Obviously the condition in which the oxygen exists in the steel has an important influence on these three characteristics. Apparently the aluminum treatment in the ladle precipitates or fixes the oxygen and possibly the nitrogen in the steel, thereby depressing their effect. During the past few years T. Swinden6 and his associates in England, in numerous papers, have reported data on both coarse-

16 :TIES OF KILLED BESSEMER STEEL grained (semikilled or rimmed) and finegrained (fully killed) bessemer steels, which fully corroborate the data presented in this paper regarding the freedom of fine-grained, fully killed bessemer steel from blue-heat brittleness, susceptibility to strain-aging, and low impact properties. Swinden's paper also points out the negligible effect of nitrogen, when under per cent, on strain-aging embrittlement when the bessemer steel is in the fine-grained, fully deoxidized condition. Previn~ls referenre has heen made tn the cleanliness and Jominy hardenability characteristics of the low-carbon killed bessemer steel. A survey of the carburizing properties of this material and investigation of specimens quenched after carburizing indicate that it compares very favorably in this respect with standard open-hearth carburizing steels of the 1020-X type, which normally contains o. 15 to 0.25 per cent carbon and 0.70 to 1.00 per cent manganese. McQuaid-Ehn tests on the finegrained open-hearth steel compared with killed bessemer steel show that the latter is somewhat finer in grain size and possesses the same degree of normality as the corresponding open-hearth steel. A survey of quenched carburized specimens by Rockwell hardness tests and hydrochloric etch for detection of soft spots showed that the carburized bessemer steel was in general less affected by soft spots than the finegrained 1020-X open-hearth steel after carburizing. In addition, the core strength of the bessemer steel is greater than that of the open-hearth steel. It is believed therefore that the low-carbon killed bessemer steels, because of their greater cleanliness and at least equally desirable carburizing characteristics, may have useful applications in this field. Results of the investigations of the killed bessemer steel described are based on at least five years of testing on thousands of heats of steel. Except for the creep tests and the hardenability and carburizing studies, the properties reported represent results on a large number of heats. There can be little doubt as to the uniformity of this type of bessemer steel compared with standard open-hearth grades. A summary of the important conclusions drawn from the work is given below: I. The yield strength and fatigue properties of killed bessemer steel are superior to those of open-hearth steel of the same tensile strength. 9 The dilrtility and tniiohn~ss nf the killed bessemer steel both at room and subzero temperature is equal to the ductility and toughness of open-hearth steel of the same tensile strength provided both steels are thoroughly killed with the same deoxidation practice. 3. The ease of welding and metallurgical effects adjacent to welds of the killed bessemer steel are better than for openhearth steel of the same tensile strength. 4. The cleanliness and carburizing qualities of killed bessemer steel are equal to those of corresponding open-hearth grades. 5. The susceptibility to strain or strainaging and the amount of blue-heat brittleness developed by killed bessemer steel is similar and approximately equal to the strain-aging or blue-heat brittleness of killed open-hearth steel of the same tensile strength, finished with the same deoxidation practice. 6. The creep strength of the fine-grained killed bessemer steel is higher than the creep strength of fine-grained open-hearth steel finished with the same deoxidation practice. The development of the killed bessemer steel represents intensive work during the past five years among most of the metallurgists and steel-plant superintendents of the National Tube Co. Mr. P. F. Mumma, superintendent of steel plant at McKeesport, and Mr. W. B. Kennedy, chief metallurgist of the same plant; Messrs.

17 E. C. WRIGHT J. E. Could and J. D. Tyson, metallurgists at the Lorain plant of the National Tube Co., and Mr. H. W. Hudson, assistant general superintendent of the National Tube Co. plant at Ellwood City, have all contributed greatly to the study of the properties of this type of steel as it has passed through operations under their supervision. The help and cooperation of these men greatly facilitated this development. I. T. Chi~man: Trans. Amer. Soc. for Metals i H. 4. Gillett: Phosphorus as an Alloy Element in Steel. Metals and Allovs - (103~) , H. C. Cross and D. E. Krause: Phosphorus as an Alloying Element in Steels tor Use at Elevated Temperatures. Metals and Alloys (1937) 8. ~$ H. W. Graham and H. K. Work: Proc. Amer. Soc. Test. Mat. (1939) 39, T. Swinden:Jnl. Iron and Steel Inst. (1936). T. Swinden and F. B. Cawley: Inst. of Marine Engrs. (1939) 51, pt. 3, 99.