Calculated Hardenability and Weldability of Carbon and Low-alloy Steels

Transcription

1 AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS Technical Publication No ~ ,. -."--, DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented by the contributor in person at the Cleveland Meeting. October 19. when an abstract of the paper w~ll be read. If this is impossible disc!~ssion in writing (z copies) may be sent to the Secretary American Institute of Mining and ~etallur~icai Eng~neers. zg West 9th Street, New York. N. Y. Unlzss special arrangement is made. discussion of this paper will close Dec Any discussion offered ther:after should preferably be in the form of a new paper. Calculated Hardenability and Weldability of Carbon and Low-alloy Steels BY C. E. JACKSON,* MEJ~BER A.I.M.E., AND G. G. LUTHER* (Cleveland Meeting, October 19) THE relationship between hardenability and weldability has been mentioned many times. The ease of making a hardness survey has led to its wide use as a criterion elements were considered to be additive, but recently Grossmann proposed a novel method of calculating the hardenability of a steel from its chemical composition. In the of weldability and with a given class or series of steels for which the ductility under weld heat-treatment is known the test is entirely suitable. However, in studying a new steel the hardness survey is of less value, since it is based on the assumption that there is a correlation between hardness and ductility. Several attempts have been made to derive indices for weldabilityl and hardenability from the chemical analysis. In most of these proposals the effects of the different Published by permission of the Navy Deuartrnent. Manuscri~t received at the office of the Institute July Z, - I ~ ~ Z. * Division of Physical Metallurgy, Naval Research Laboratory. Anacostia Station. Washington. D. C. 1 References are at the end of the paper. method proposed, a steel is considered as having a base hardenability due to its carbon content and grain size, and this base hardenability is multiplied by a factor for each chemical element present. The final product is the hardenability. The purpose of the present study is to point out the possible relationship between hardenability and the effect of welding on hardness and ductility for a number of plain carbon and low-alloy steels. The cornposition of the steels reported in this paper as determined by chemical analysis is given in Table I. To eliminate the effect of variations in finishing temperatures, the -in. plates were normalized after heating for one hour at 16j0 F. in a controlled-atmosphere furnace. Copyright. 19. by the American Institute of Mining and Metallurgical Engineers. Inc. METALS TECHNOLOGY. October 19 Printed in U. S. A.

2 CALCULATED HARDENABILITY AND WELDABILITY OF STEELS In making colnparative tests, it is essential to hold conditions as constant as possible; hence full automatic welding control was used. The electrodes, all supplied by Single bead welds were deposited transverse to the direction of rolli~lg on sections of >$in. plate, 6 in. wide by 7 in. long, by using full automatic welding. A uniform FIG..-MJCROSTRUCTURE AT FUSION LINE SRO\VING A SERIES OF KNOOP X roo INDENTATIONS. one manufacturer, were of mild steel, welding technique was used, with 17- heavy coated, reversed polarity (gradeea, amp. current, an arc voltage of 6 volts, class I) with B{6-in. diameter core. and a speed of travel of 6 in, per min. C 1 Mn TABLE I.-Composition 1 Si 1 S 1 P I Ni Composition. Per Cent oj Steels Determined by Chemual Analysis " ' ~ o O.IZ~ IqO ~ o ' ' ~ o.orz Cu o a Experimental steels. Cr Grain Size at Fusion Line Calculated ~ ~ ~ d ~ ~ ~ In I I

3 C. E.. JACKSON AND G. G. LUTHER Knoop and Vickers hardness numbers were determined on sections cut transverse to the bead weld (Fig. I). For the Knoop hardness indents, a load of 0. kg. The precision of the Knoop method is such that it is possible to detect changes in hardness that cannot be measured by the usual Vickers hardness test methods. CALCULATED HARDENABLITY- INCHES FIG..-RELATION OP MAXIMUM KNOOP HARDNESS AND CALCULATED HARDENABILITY. was used. Indentations were made with a A photograph at loo times magnification separation of not more than 0.1 mm. of the microstructure at the fusion line is (0.00 in.) across the heat-affected zone shown in Fig.. The enlarged ferriteand particular attention was given to the pearlite grain size is well defined. The fusion line. The point of maximum hardness A.S.T.M. grain size for the structure that usually occurred 0.1 to 0. mm. away from shows the greatest grain growth was the deposited metal into the base metal. determined (Table I), since it is this grain

4 CALCULATED HARDENABILITY AND WELDABILITY OF STEELS size that influences the hardenability and lower surface to obtain a specimen 0.7 in. mechanical behavior of the fusion zone. thick, with a ground finish on both faces. The results obtained from calculation of The specimens were etched in a per cent the hardenability of the zone of grain nital solution and the location of the V- GALGULATED HARDENABILITY - INCHES FIG..-RELATION OF ANGLE AT MAXIMUM LOAD FOR SLOW-BEND SPECIMENS AKD CALCULATED HARDENABILITY. growth, using methods proposed by Grossmann, are also given in Table I. In some steels that yield an acicular structure in the heat-affected zone, difficulty may be encountered in determining the ferritepearlite grain size at the fusion zone. As a measure of the effect of welding on ductility, strips I 6 in. wide were cut transverse to the direction of the bead weld. Only sufficient metal was removed from the top surface to eliminate surface inegularities. Material was then machined from the notch was determined by scribing a line on the side of the specimen. A standard Izodtype of V-notch was machined with its apex tangent to the fusion line between the weld metal and the plate material (Fig. ). For comparison, an identical specimen was prepared and tested from the plate material. The specimens were bent to failure with the notched face in tension in a test jig. The angle at maximum load was taken as a measure of ductility for each test specimen.

5 C. E. JACKSON AND G. G. LUTHER The relation of hardenability as determined by calculation from the chemical composition to weldability test data is presented in Table and Figs. and. The correlation between the calculated hardenability and maximum hardness as determined by the Knoop method is excellent for the experimental steels, in which the minor constituents were held to a mini- TABLE.-Relation of Hardenability to Weldability Calculated Hardenability. In. Hardness of ~~~d weld Vickers. Knoop. 10 Kg. 0. Kg. V-notched Slow-bend Angle at Maximurn Load. Deg Bead 'late 1 Weld factors, such as the type and number of nonmetallic inclusions or other segregation present in the steel. The use of calculated hardenability, or may we say calculated weldability, will increase in its usefulness as additional data for the various welding steels become available. Probably it will be necessary to determine new weldability factors for alloying additions-and it is entirely possible that these factors for the welding thermal cycle will not be in strict agreement with the factors for full hardening. Care must be taken in applying the calculation to any steel, as the combination of mechanical properties obtained for an alloy steel may show unusually high ductility, although the maximum hardness determined may conform to the general prediction. Additional data should be obtained in order to determine more accurately the limits for the application of calculated hardenability. Actual measurements of hardenability by using a Jominy type specimen or a quencbed taper bar will also be useful. Tentatively, a limit of hardenability of a I.o-in. round in an "ideal quench" (grain size ) seems to be a limit for straightforward welding in plain carbon and low-alloy steels. mum. This correlation is by no means as good for the commercial steels, in which there is less control of the minor constituents and for which complete chemical analyses were not made. This will explain the fact that in most cases the maximum hardness reported is higher than that which might be predicted from the calculated hardenability. The relationship between the calculated hardenability and weld ductility as measured by the \'-notched slow-bend specimen is not as regular as that for the maximum hardness. This is to be expected, as the ductility will also be dependent upon other The authors wish to express their indebtedness to F. M. Walters, of the Division of Physical Metallurgy, Naval ResearchLaboratory, for his support and counsel in this work. Assistance in test work given by M. A. Pugacz, of the Division of Physical Metallurgy, Naval Research Laboratory, is also gratefully acknowledged.. I. J. Dearden and H. O'h'eill: A Guide to the Selection and Welding of Low-alloy Structural Steels. Trans. Inst. of Welding (190) M. A. Grossmann: Hardenability Calculated from Chemical Composition. A.I.M.E. Tech. Pub. 17 (Metals Tech.. June 19). C. E. Jacksonand G. G. Luther: Weldability Tests of Xickel Steels. Research Supplement. The Welding Jnl. (Oct. 1~1) 0.