Abrasion Resistance of Iron-Based Hardfacing Alloys

Transcription

1 Abrasion Resistance of Iron-Based Hardfacing Alloys Carbon content, not hardness, is the most important factor determining resistance to low-stress abrasion BY D. J. KOTECKI AND J. S. OGBORN ABSTRACT. A study was undertaken of numerous iron-based hardfacing compositions as arc weld deposits (SMA, FCA and SA) to evaluate quantitatively their low-stress abrasion resistance as a function of composition and hardness. The method of evaluation is the ASTM G65, Procedure A, dry sand/rubber wheel test, determining weight loss after a fixed number of revolutions of the wheel. A broad spectrum of deposits was prepared from commercial and experimental electrodes. One-, two-, and four-layer cladding permitted examination of dilution effects. The compositions studied included buildup alloys, martensitic deposits, austenitic manganese, primary austenite with austenite-carbide eutectic, near-eutectic austenite-carbide, and primary carbides with austenite-carbide eutectic. Composition and abrasion resistance are determined generally on one, two, and four layers of hardfacing on mild steel. In all, about 00 hardfacing deposits were evaluated. The most important variable in determining low-stress abrasion resistance was found to be carbon content. Above about % carbon, numerous primary carbides are obtained and abrasion resistance is greatest. Hardness and chromium content (alloy content) have, at best, secondary effects on abrasion resistance. Dilution has an important effect, often causing an alloy that consists of primary carbides in multiple layers to be primary austenite in the first layer, with resulting inferior abrasion resistance. Introduction There are many forms of abrasion, in- D. J. KOTECKI and J. S. OGBORN are with The Lincoln Electric Co., Cleveland, Ohio. Paper presented at the AWS 7th Annual Meeting, April 5-9, 993, Houston, Tex. cluding low stress (force applied to abrading particles is not sufficient to crush or fracture the particles), high stress (force applied to abrading particles also crushes the particles), dry abrasion, wet abrasion, high-velocity particle impingement (wet or dry), etc. (Ref. ). Therefore, in grading a hardfacing alloy for abrasion resistance, it is necessary to specify the form of abrasion to be resisted. Recognizing this, ASTM (Ref. ) has developed a number of standard test methods for various forms of abrasion, including ASTM G65 (low-stress abrasion), ASTM G99 (high-stress abrasion), ASTM G05 (wet low-stress abrasion), ASTM G75 (slurry abrasion), ASTM G76 (solid particle impingement), and ASTM G73 (liquid impingement). Even within just the iron-based family of hardfacing alloys there are a number of microstructures which provide varying degrees of resistance to abrasion, including ferrite/bainite, martensite, austenite, and carbides (Ref. 3). In most cases, carbides provide highest abrasion resistance within the iron-based system. As long ago as 99, Haworth (Ref. ) reported KEY WORDS Low Stress Abrasion Resistance Iron-Based Hardfacing Alloys Carbon Content Microstructure Dilution Effects FCAW Submerged Arc Welds that carbon is the most important element in providing abrasion resistance to irons and steels, though the tests Haworth used were not standardized. Haworth further concluded that other elements are of secondary importance, and that macrohardness of an iron or steel showed no correlation with abrasion resistance. However, quantity and hardness of microconstituents did correlate with abrasion resistance. Avery and Chapin (Ref. 5), using a nonstandardized wet sand abrasion test, concluded that, among other things, while additional alloying with molybdenum, tungsten, or vanadium increased the hot hardness of high-chromium irons, the added expense was not justified by improved abrasion resistance. Admittedly, both of these works presented limited data using nonstandardized testing, but they drew consistent conclusions. Despite these works dating back as much as 0 years, abrasion-resisting hardfacing alloys today still tend to be promoted based upon hardness and/or alloy content, rather than upon carbon content and microstructure. Quaas (Ref. 6) reported dry sand-rubber wheel abrasion tests (still not standardized), which appear to show increasing abrasion resistance with increasing carbon content of highchromium irons. He also showed carbide morphology changes with welding parameter changes. However, he only reported a single undiluted weld metal composition for any given electrode tested, although it was often tested under several welding conditions. This serves to highlight a shortcoming of much hardfacing data: a tendency to report only an all-weld-metal composition for a given electrode, rather than to report the actual deposit composition of the surface tested. It should be recognized that analysis of hardfacing deposits is difficult because of the need to prepare chips WELDING RESEARCH SUPPLEMENT I 69-s

2 Fig. -- Map of hardl~cing alloy compositions and microstructures. Alloy includes Cr, Mn, Mo, Ni, Nb, V, W, Ti, AI. from materials that are deliberately resistant to machining. Microstructures of Iron-Based Hardfacing Alloys Commercially available iron-based hardfacing weld deposits can be categorized (Ref. 3) according to microstructure as: ) ferritic/bainitic buildup alloys () ) martensitic alloys () 3) mixed martensitic/austenitic alloys (MA) ) austenitic manganese alloys () 5) primary austenite with austenite-carbide eutectic () 6) near-eutectic austenite-carbide alloys () 7) primary carbides with austenite-carbide eutectic (). In addition, martensitic alloys can be subdivided according to whether they are low-alloy martensite, tool steel martensite (with secondary hardening capability in PWHT), or stainless steel martensite (with corrosion resistance). Austenitic manganese alloys can be subdivided into ordinary alloys and premium alloys (which contain enough alloy to produce stable austenite even in the diluted first layer over mild steel). And primary carbide alloys can be subdivided into ordinary chromium carbides, premium carbides (including considerable Mo, Nb, V, W, and/or Ti), and extra-high carbides (which will produce primary carbides even in a diluted first layer on mild steel). Figure shows a "map" of hardfacing alloy content and carbon content with ranges for most commercial iron-based alloy microstructures according to the above categorization scheme. Figures -8 show examples of the seven main iron-based hardfacing alloy microstructure types, taken from the present study. The composition given with each microstructure is for the actual weld deposit shown. Field Experiences As noted above, hardfacing alloys are often promoted for an application based upon hardness or alloy content. But field experiences do not support this approach. For example, it is well known that 50 to 55 Rockwell C hardness can be obtained with at least three very different microstructures: low-alloy martensite, work-hardened austenitic manganese, and near-eutectic austenite-carbide. But in low-stress abrasion, near-eutectic austenite-carbide is generally found to out-perform the other two microstructures by a considerable margin. Similarly, a more highly alloyed chromium carbide composition will not necessarily outperform a lower alloyed chromium carbide. In fact, the reverse is usually true if the lower alloy is higher in carbon. This is in line with Haworth's paper (Ref. ), where carbon is considered to be the most important element determining abrasion resistance of ironbased alloys. Experimental Program and Results To provide a sound foundation for recommending iron-based hardfacing alloys to users of hardfacing, a program was undertaken to evaluate numerous alloys as arc welding deposits specifically for lowstress abrasion resistance. For this, the ASTM G65 Procedure A was chosen. This is a well-standardized test method (first published in 980) that uses dry quartz sand of tightly limited particle size, 95% minimum in the U.S. sieve size range -50 to +70 (-300 to + microns), flowing in a thin layer at 300 to 00 g/min between the test piece and a hard rubber wheel 9 mm (9 in.) in diameter. The test apparatus used is shown in Figs. 9 and 0. Test coupons after testing are shown in Fig.. The force applied pressing the test piece against the wheel is 30 N (30 Ib) and the test is car-! ~ ~ ~,..>i ~ Fi~. -- Typical microstructure of ~erriticl'bainitic buildup iron-based hard,acing alloy, 650X, % nital. Composiffon of e ample: O. % C, I. 5% Mn, I A% Cr. Left; slow cooling conditions, Rockwell C 0; Right: fast cooling conditions, Rockwell C s AUGUST 995

3 Fig Typical microslructure of martensitic iron-based hardt~cinq alloy, 650X, % nital. Composition of example: 0.% C,.7% Mn,.% Cr. ried out for 6000 revolutions of the wheel at 00 rpm. The test piece is weighed before and after the test, and the weight loss can be used directly or converted to volume loss. In the present study, weight loss is reported. For all of the hardfacing tests, the base plate was ASTM A36 steel, or equivalent. In most cases, it was in the form of /- in. (.7-mm) thick by 3-in. (75-mm) wide hot rolled bar about in. (360 mm) in length. The first layer of deposit was made the full length of the bar. A second layer was deposited over two-thirds of the bar length, and the third and fourth layers were deposited over the last third of the length. All welding was done using stringer beads, with no initial preheat and a maximum interpass temperature of 00 F (05 C). Then approximately 3-in. (75-mm) long coupons were cut from the one-, two-, and four-layer deposits. These were surface ground smooth and weighed to the nearest 0.0 g. After conducting the ASTM G65 Procedure A test, the coupon was weighed again, and the weight loss was calculated. In most cases, at least two side-by-side tests were done on a given coupon, and the results were averaged. For comparison purposes, a few tests were also done on A36 steel base metal, higher carbon steel base metal, and a cast iron weld deposit on gray cast iron base metal. It should be noted that two variances were taken with the ASTM G65 Procedure A. The first is in the degree of sensitivity used in weight loss determinations. G65A requires weight sensitivity of g, which is an order of magnitude more sensitive than that used herein, but the sensitivity of 0.0 g used is quite sufficient Fig. -- Typical microstructure of mixed martensitic-austenitic ironbased hardfacing alloy, 67X, Vilella's etch. Composition of example: 0.9% C, 0.% Mn, 9.5% Cr, 0.6% Mo. Islands of unternpered martensite are surrounded by extensive retained austenite. to show significant differences in abrasion resistance among the deposits. Secondly, G65A requires determination of volume loss, not weight loss. However, for comparison purposes, when the materials being compared are all iron-based with similar densities, weight loss leads to the same conclusions as does volume loss. Weight loss would clearly be inappropriate for comparison of tungsten carbide to chromium carbide deposits, for example, because the densities are quite different, but that is not the case herein. For all hardfacing deposits evaluated, the actual hardfacing composition on the surface tested was determined by machining chips and analyzing. In most cases, a given hardfacing electrode would be tested in one, two, and four layers of surfacing on mild steel. This resulted in three distinct deposit composi- o e ~ " ".*.:. Z"' Fig Typical microstructure of austenitic manganese iron-based hardfacing alloy, 60X, 5% nital followed by 5% HCI. Composition of example: 0.6% C, 3% Mn, 0.5% Ni, 5% Or. The parallel lines within each austenite grain are twin boundaries. Fig Typical microstructure of primary austenite with austenite-carbide eutectic iron-based hardfacing alloy, 650X, 5% nital. Composition of example:.% C,.3% Mn, I.% Cr, 0.6% AL The white areas are primary austenite. The mottled areas are austenite-carbide eutectic. WELDING RESEARCH SUPPLEMENT I 7-s

4 Fig Typical microstructure of near-eutectic austenite-carbide ironbased hardfacing alloy, 650X, Vilella's etch. Composition of example: 3.5% C, 5% Cr. Fig Typical microstructure of primary carbide with austenite-carbide eutectic iron-based hardfacing alloy, 60X, Vilella's etch. Composition of example:.3% C,.6% Mn, 5.% Cr. Hexagonal shapes are CrzC 3 primary carbides. The mottled matrix is austenite-carbide eutectic. tions due to dilution effects. In several cases, the complete chemical analysis for all elements known to be present in a given hardfacing consumable was not run on all three wear pads, but the missing element analysis can be inferred from the data for the other pads. Also, with a number of the chromium carbide deposits, manganese and silicon were not analyzed even though these elements are known to be present in incidental amounts. For the lower alloyed deposits which produce either ferrite/bainite or martensite microstructures, chemical analysis for all elements except carbon was obtained by atomic emission spectrophotometry. All carbon analyses were performed by a fusion instrument. Higher alloyed deposits were analyzed by wet chemical analysis. The approximately 00 deposit compositions evaluated by ASTM G65A testing are given in Table, along with an indication of the microstructure of each. When two or three successive tests have the same test code, the layer number differentiates among the two or three compositions from the same consumable having different dilutions with the mild steel base metal. In a few cases, the wear test sample was inadvertently discarded without hardness testing, so no Rockwell C number is given in Table. In such cases, the hardness can be inferred from that of a similar composition elsewhere in Table. The G65A test results for the A36 and other carbon steel base metals and for the gray cast iron weld deposit are also included in Table. The microstructure of any given deposit in Table can be determined by locating the carbon and total alloy content on the map of hardfacing alloy compositions and microstructures (Fig. ), as well Fig ASTM G65 Procedure A abrasion test apparatus overview. Fig ASTM G65 Procedure A abrasion test apparatus. Closeup of test coupon pressed against rotating rubber wheel while a thin stream of sand flows between the wheel and the coupon. 7-s I AUGUST 995

5 Fig. -- ASTM G65 Procedure A test hardfacing test coupons after test. The specimen on the left is a martensitic deposit, while that on the right is a near-eutectic austenite-carbide deposit. Both are about 55 Rockwell C hardness. Note the larger wear scars on the martensitic deposit. Fig. -- Microstructure of Test Code 0CCR, consisting of M3C primary carbides in a matrix of austenite-carbide eutectic, 6X, Vilella's etch. Note the absence of the hexagonal shape of the primary carbides, as compared to those of Fig. 8. as by metallographic examination. Enough metallographic examination was done to verify the map. This permits appreciation of dilution effects. For example, for Test Code W, the first layer of hardfacing contains about.5% C and about % alloy, mostly chromium. The microstructure is therefore primary austenite with austenite-carbide eutectic, and the abrasion resistance is not very high, with 0.99-g weight loss in the G65A test. For the same consumable, the second layer contains 3.6% C and about 0% alloy, so that the microstructure is nearly all austenite-carbide eutectic, and the abrasion resistance is better than that of the first layer, 0.-g weight loss. The fourth layer of W contains about.5% C and about 5% alloy, so that the microstructure is primary carbides with austenite-carbide eutectic, and abrasion resistance is still better, 0. g weight loss. Other observations can be made as well. For example, Test Code W9 is a tool steel martensite. It is about 60 Rockwell C hardness in first, second, and fourth layers. Its abrasion resistance is no better in the fourth layer than in the first layer. It is as hard as, or harder than, W in first, second, or fourth layer, but its abrasion resistance is only as good as that of W in the first layer. In the fourth layer, the tool steel martensite wears at about five times the rate of the primary carbide microstructure despite similar hardness. This serves to emphasize that microstructure is more important than hardness in determining abrasion resistance. Another observation that can be made is that rather pricey premium carbide consumables, such as W8, W3, or W, provide little or no improvement in G65A abrasion resistance as compared to a relatively low-cost primary carbide consumable, such as W7, when the carbon level is similar. A near-eutectic consumable with about 30% Cr, such as W3 or W53, does not resist abrasion as well as a lower chromium, higher carbon primary carbide, such as W (0% Cr) or even 5CCR (6.% Cr). However, it is possible to reduce the chromium low enough that primary chromium carbides are not formed, even at above % C, and the abrasion resistance begins to suffer. Examples of this are experimental compositions 0CCR (0.% Cr,.% C) and 0CCR (8.7% Cr,.0% C), whose abrasion resistances are not as good as that of the other primary carbide compositions. The microstructure of 0CCR is shown in Fig., and it can be seen that the primary carbides are not hexagonalshaped like those in Fig. 8, indicating a different crystal structure from that of Cr7C 3. The C-Cr-Fe ternary phase diagram (Ref. 7) indicates that M3C (essentially Fe3C with some substitution of Cr for Fe in the carbide crystal lattice) will be the primary carbide phase below 5% Cr, while M7C 3 (essentially CrTC 3 with some substitution of Fe for Cr in the carbide crystal lattice) will be the primary carbide phase above 5% Cr. Cr7C 3 is much harder (approximately 00 DPH) than Fe3C (approximately 300 DPH), Ref. 8. The existence of hexagonal-shaped primary carbides was confirmed metallographically in the 5CCR deposit. In the extreme, a gray cast iron deposit (W36 in Table ), with high carbon but no chromium, has very poor abrasion re- sistance, comparable to that of carbon steel base metal, or even worse. Analysis of Results The observations made above are interesting one-on-one comparisons, but it is more useful to look for global trends in the data of Table. As was noted above in the Field Experiences section, hardfacing consumables are often promoted on the basis of deposit hardness as an indicator of abrasion resistance. The results in Table can be used to examine this approach by plotting G65A weight loss vs. deposit hardness. This is done in Fig. 3. Where a hardness value is absent for a given composition in Table, the test results do not appear in Fig. 3. While a correlation of lower weight loss (greater abrasion resistance) with increasing hardness is noticeable in Fig. 9, the scatter in results viewed this way is huge. One has to conclude that hardness is a very poor predictor of abrasion resistance. A different symbol is used for each of the seven microstructural types in Fig. 3, so some clustering of the data according to microstructure is apparent in this figure. In particular, the primary carbide microstructures are all found near the bottom of the figure (lowest weight loss or greatest abrasion resistance). Similarly, it was noted in the Field Experiences section that hardfacing consumables are sometimes promoted on the basis of alloy content, with higher alloy content claimed to provide greater abrasion resistance. This could be evaluated by plotting the data of Table in several ways. One way is to plot the G65A results against deposit chromium con- WELDING RESEARCH SUPPLEMENT I 73-s

6 Table -- ASTM G-65 Practice A Test Results: Dry Sand, Rubber Wheel, 6000 Revolutions, 30-b Force Hard- Test Welding ness, Code Process Layer Rc C Mn Si Cr Mo V Nb W Ni AI Average Weight Loss, g Microstructure Type (a) C W W W W W W W W W W W W W W W W W W W W W W W W Wl Wl Wl Wl I 0.5 Wl Wl W Wl W Wl Wl Wl W W Wl Wl Wl Wl Wl Wl W Wl 7 FCAW W7 FCAW Wl 7 FCAW Wl 8 FCAW Wl 8 FCAW Wl 8 FCAW Wl 9 FCAW Wl 9 FCAW Wl 9 FCAW W0 FCAW W0 FCAW W0 FCAW W FCAW W FCAW W FCAW W W W W W W W , s I AUGUST 995

7 W W W5 W5 W5 W6 W6 W6 W7 W7 W7 W8 W8 W8 W9 W9 W9 W3 W3 W3 W3 W3 W3 W33 W33 W33 W39 W39 W39 W0 W0 W0 W W W W W W W3 W3 W3 W W W W5 W5 W5 W6 W6 W6 W7 W7 W7 W8 W8 W8 W9 W9 W9 W50 W50 W50 W5 W5 W5 W5 W5 W5 W53 W53 W53 W56 W56 W56 W57 FCAW O O O.O O O MA O O O O WELDING RESEARCH SUPPLEMENT I 75-S

8 W57 FCAW W57 FCAW W58 W58 W58 W83 FCAW W83 FCAW W8 FCAW W8 FCAW W85 FCAW W85 FCAW W86 FCAW W86 FCAW Wl FCAW Wl FCAW Wl FCAW W FCAW W FCAW W FCAW RW FCAW RW FCAW RW FCAW W8 Wl 8 W8 W9 W9 W9 W30 W30 W30 5G FCAW 69G FCAW 70G FCAW 5PN FCAW 5PN FCAW 6PN FCAW 6PN FCAW Wl 3 Wl 3 Wl 3 W3 W33 W3 W3 W3 W35 W35 Wl 35 L60 FCAW L60 FCAW FCAW 35CCR FCAW 330CCR FCAW 0CCR FCAW 0CCR FCAW 330CCR FCAW 5CCR FCAW 35CCR FCAW H0 FCAW H0 FCAW H0 FCAW W36 FCAW W7 W7 W7 A36 Base Metal 030 Base Metal 00 Base Metal RB RB R GRAY IRON MA MA FP FP FP Microstructure types are: = ferrite/bainite FP = ferrite/pearlite = martensite MA = martensite and austenite mixed = austenitic manganese = primary austenite with euctectic austenite-carbide = near-eutectic austenite-carbide = primary carbides with austenite-carbide eutectic 76-s I AUGUST 995

9 tent. This is done in Fig.. Where a chromium value is absent for a given composition in Table, the test results do not appear in Fig.. If anything, the apparent slight correlation of lower weight loss (greater abrasion resistance) with greater chromium content is even weaker than the correlation shown in Fig. 3 for hardness. The scatter is even more severe. Some clustering of the data for particular microstructure types is also evident in this figure. Many other plots of the data of Table were similarly unsuccessful in exhibiting a good correlation of G65A weight loss with another factor. However, a very good correlation was found of G65A weight loss with deposit carbon content, as shown in Fig. 5. Where a carbon value is absent for a given composition in Table, the test results do not appear in Fig. 5. The correlation of reduced weight loss (greater abrasion resistance) with increasing carbon content is unmistakable, and scatter is relatively small, especially at higher carbon contents. This is by far the most significant global correlation found in the data of Table. Discussion of Results Figure 5 is the most important finding of this study. It shows the effect of carbon on abrasion resistance. But there is also a microstructure effect within Fig. 5. In view of Fig., it should be no surprise that each microstructure type is clustered in a specific carbon range. In the low-carbon region of Fig. 5 (below % C), there appear to be two separate trend lines within the clustered data. Carbon contents between about 0.3% and a bit over.0% can belong to martensite microstructures or to austenitic manganese. The martensites (low alloy, tool steel, or stainless) are considerably harder than austenitic manganese. Since both are essentially free of harder carbide particles, it is the matrix alone which resists abrasion, and martensite has a clear advantage over austenite. With increasing carbon beyond %, significant austenite-carbide eutectic doesn't appear until about % C is exceeded. The alloy compositions in the to % C range show no improvement in abrasion resistance over that of high-carbon martensite -- if anything, they are less abrasion resistant than the martensite. In the to 3% carbon range, there is more austenite-carbide eutectic, and less primary austenite, so the abrasion resistance improves slightly, but the primary austenite microstructure is little better than martensite. In the 3 to % carbon range, where the microstructure is near- 3.5 o 3.0 hn ~.5 < "0 ) e~ u.0.5 [.-. ~ o.5 < o o ~v o ug I :v y! vv o v~vv vv ]~B + MA o <, I ol o v vv v vv vv O00.~vVVO ~ o ~ 0 o -~o" o',c, ~o 0 ~ v v 9 v o ~.~.. 0 ~@ ee oe [] - 8 ' '0 ' 3'0 '0 5'0 6'0 70 Deposit Hardness, Rockwell C Fig ASTM G65 Procedure A weight loss vs. deposit hardness for all compositions in Table. While a slight trend toward lower weight loss with greater hardness is apparent, the scatter is very large. 3.5 o 3.0 ~.5 ~.5.0 ~.0 i I [.-, 0.5 rj3.<: ilj j Go o / v v I " ;, ",> v v v 0 o ooo o o oo ooo o 0 * MA o o 0 ~. 0 0 <> ~e 0 "a 0 []. OOo o ".o- J i 5 i i i 5 '0 '0 '5 3~0 35 Deposit Percent Chromium Fig. -- ASTM G65 Procedure A weight loss vs. deposit chromium content for all compositions of Table l. While a slight trend toward lower weight loss with greater chromium content is apparent, the scatter is very large. eutectic, abrasion resistance improves with increasing carbon. Above about 3.5% carbon, primary carbides start to appear, but they remain rather widely dispersed and the abrasion resistance is dominated by the austenite-carbide eutectic. From about % carbon upward, the abrasion resistance is dominated by primary carbides, with high abrasion resistance, and there is slight further improvement above 5% carbon. Figure, despite the very large scatter observed, points out that, within a given microstructure class, the chromium content has little, if any, effect on abrasion resistance. For example, for the primary austenite with austenite-carbide eutectic microstructure type (), or for the near-eutectic microstructure type (), or for the primary carbide with austenite-carbide eutectic microstructure type (), there is little or no benefit as regards G65A abrasion resistance for highly alloyed (high chromium) compositions as compared to lower alloyed compositions (although there may be benefits of higher chromium when corrosion becomes a factor, such as in wet abrasion). For a given microstructure type, about 6% Cr is just as abrasion re- WELDING RESEARCH SUPPLEMENT I 77oS

10 MA o ~.5 o ~.0,,,, o.5 l_ -.l,-,p, o 0, o ~ o ~.0,,~ * ~ ~ "", -.. O0 0 C' ~0.5 " o O.Oo ' ' ' ' s' ' Deposit Percent Carbon Fig ASTM G65 Procedure A weight loss vs. deposit carbon content for all compositions of Table. The trend of lower weight loss with greater carbon content is unmistakable. Scatter is very small above.% carbon. sistant as is 3% Cr. It is microstructure that determines abrasion resistance, and it is in turn carbon content that largely (but not entirely) determines microstructure -- higher chromium does tend to shift the eutectic composition in the ternary Fe-Cr-C alloy system towards somewhat lower carbon content (Ref. 7). The data of Table I serve rather well to point out differences i n m icrostructu re that can occur with a given hardfacing consumable as a result of dilution. Dilution can affect both carbon and alloy content of the first (and second) layer of hardfacing. Test Codes W and W3 are both examples where consumables, which produce primary carbides in the fourth layer on mild steel, due to dilution of carbon, result in near-eutectic in the second layer and primary austenite in the first layer. If the high abrasion resistance of primary carbides is needed in the first layer of hardfacing on mild steel, then a very high carbon consumable, such as W8, should be chosen, or the welding procedure must be modified with an ordinary carbon primary carbide consumable to limit dilution. Heavy overlap of beads in one layer can often accomplish this. A change in welding process to one which provides lower dilution can also be made sometimes. There are examples where dilution of alloying elements, rather than of carbon, affects the microstructure of the first layer of hardfacing on carbon steel. Test Codes W0 and W are both cases where the o first layer of a nominally austenitic manganese consumable deposit on carbon steel turned out to be martensite due to dilution of alloying elements. This can be seen not only from the composition of the first layer and reference to Fig. I, but also from the hardness of the first layer. Austenitic manganese hardfacing is typically 0 to 5 Rockwell C in the aswelded condition, but the first layer of both W0 and W is 50 Rockwell C or higher, clearly martensitic. It is fairly common experience that such a first layer will crack under deformation, or will separate from the base metal along the weld interface when multiple layers are applied. This is one reason that premium austenitic manganese consumables, such as W, W, W56, and W85 exist. The higher alloyed consumable produces stable austenite even in the diluted first layer on carbon steel, so that a high toughness, impact resistant deposit results. The data of Table I show negligible difference in low-stress abrasion resistance between primary carbide alloys where only chromium is present to form carbides, and primary carbide alloys, which include more expensive carbide formers such as Nb, Ti, V, or W, at the same carbon level. However, abrasion can also occur under high-stress conditions, which fracture abrasive particles. In high-stress abrasion, the more expensive "premium" carbides of Nb, Ti, V, and W can show better abrasion resistance than chromium carbides. Conclusions This study has produced a considerable amount of data which allow comparisons of low-stress abrasion resistance among a large number of iron-based hardfacing alloys. These data lead to the following conclusions: I) Microstructure, not hardness, is the most important factor determining lowstress abrasion resistance of iron-based hardfacing alloys. ) The most abrasion-resistant microstructure in these alloys is primary carbide with austenite-carbide eutectic. 3) The second most abrasion-resistant microstructure in these alloys is near-eutectic austenite-carbide. ) Carbon is the most important element determining microstructure, and therefore abrasion resistance, of ironbased hardfacing alloys. Carbon content above %, with chromium content above 6%, will produce a primary carbide with austenite-carbide eutectic hardfacing microstructure. 5) Dilution from the base metal can cause the first (and even the second) layer of hardfacing to have a different microstructure from that of very low-dilution or multiple-layer deposits. With dilution effects, hardfacing properties may not be as expected. 6) Dilution effects can be overcome by selecting consumables of higher carbon (and/or higher alloy) to provide the desired microstructure in a diluted deposit. References. Budinski, K. G Surface Engineering for Wear Resistance, Chapter, Prentice- Hall, Englewood Cliffs, N. J.. Annual Book of ASTM Standards, 99 edition, Vol. 3.0, American Society for Testing and Materials, Philadelphia, Pa. 3. Kotecki, D. J. 99. Hardfacing benefits maintenance and repair welding. Welding Journal 7 ( ): Haworth, R. D., Jr. 99. The abrasion resistance of metals. Transactions of the A.S.M. : Avery, H. S., and Chapin, H. J. 95. Hardfacing alloys of the chromium-carbide type. Welding Journa3 (0): Quaas, J. F Hardfacing international. Welding Journal 9(3): Metals Handbook, 8th edition, Vol. 8, 973, p. 0, ASM International, Materials Park, Ohio. 8. Metals Handbook, 9th edition, Vol. 6, 983. p. 777, ASM International, Materials Park, Ohio. 78-s I AUGUST 995