Microstructure and Wear Characteristics of Hypoeutectic, Eutectic and Hypereutectic (Cr,Fe) 23 C 6 Carbides in Hardfacing Alloys

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1 Materials Transactins, Vl. 48, N. 9 (2007) pp t 2328 Special Issue n Slidificatin Science and Prcessing fr Advanced Materials #2007 The Japan Institute f Metals EXPRESS REGULAR ARTICLE Micrstructure and Wear Characteristics f Hypeutectic, Eutectic and Hypereutectic (Cr,Fe) 23 C 6 Carbides in Hardfacing Allys Che Wei Ku*, Chieh Fan*, Sheng Ha Wu* and Weite Wu Natinal Chung Hsing University, Department f Materials Science and Engineering, 250 Ku Kuang Rad, Taichung 402, Taiwan, R. O. China This study investigates the micrstructure and wear characteristics f hypeutectic, eutectic and hypereutectic (Cr,Fe) 23 C 6 carbides in hardfacing ally cladding by using gas tungsten arc welding (GTAW). Chrmium and graphite ally fillers were used t clad ASTM A36 steel substrates. These catings were specially designed t vary the size and prprtin f the (Cr,Fe) 23 C 6 carbides that are present in the micrstructure at rm temperature. Depending n the amunt f graphite used in the ally filler, a hypeutectic, eutectic r hypereutectic micrstructure was btained n the cated surface. The wear behavir indicated that the abrasive wear resistance is nt nly simply related t the hardness f the depsit but that it is als determined by the carbides and matrix structure f the cating. The hypereutectic carbides have the largest (Cr,Fe) 23 C 6 carbide cntent and the maximum hardness; hwever, they shw the wrst wear resistance due t the large particles were dug ut during wearing. [di: /matertrans.mb200716] (Received May 2, 2007; Accepted July 12, 2007; Published August 25, 2007) Keywrds: hardfacing, welding, chrmium carbide, abrasive wear 1. Intrductin Hardfacing allys are used under the cnditins f extreme ersin and abrasive wear. The exceptinal abrasive and ersive wear resistance characteristics f hardfacing allys results primarily frm their high vlume fractin f hard carbides, althugh the tughness f the matrix als cntributes t the wear resistance. Observatins f Fe-Cr-C allys f Fe-Cr-C ally micrstructures have shwn that these types f materials cntain hypeutectic, eutectic and hypereutectic structures. 1) M 7 C 3 primary carbides are frmed in large amunts at higher carbn cncentratins. These types f micrstructures have gd wear resistance prperties. 2,3) Fe-1235Cr-C white cast irns are used as hardfacing allys; these allys have a high hardness value due t their high M 7 C 3 cntent. 4 7) M 7 C 3 is surrunded by austenite, which is relatively sft in cmparisn t the extremely hard M 7 C 3. Cnsequently, cracks prpagate alng the interface between austenite and M 7 C 3. This causes a serius prblem fr hardfacing materials. Fe-28Cr-C hardfacing allys are als used cmmercially fr cmpnents that are subject t harsh abrasive cnditins. The large amunts f carbides present in their micrstructures cnsist f M 23 C 6. They can be described as cmpsites with large and extremely hard carbides in a sfter bdycentered-cubic (bcc) Cr-Fe ally matrix. Hardfacing allys btained using high-energy density surces such as electrn beam welding, plasma arc and lasers have been widely applied in the industry t enhance the wear and crrsin resistance f the surfaces f underlying materials. 8 10) The criteria used fr selecting the welding surface fr wear and hardness applicatins depends n the features f its micrstructure. In this investigatin, the gas tungsten arc welding (GTAW) prcess is emplyed t frm a Fe-Cr-C hard surface with high *Graduate Student, Natinal Chung Hsing University chrmium cntent ver the ASME A36 steel with chrmium and graphite ally fillers. The changes in the micrstructures and the wear characteristics f the cated surfaces were studied in detail by using an ptical micrscpe (OM), a field emissin scanning electrn micrscpe (FE-SEM), an electrn prbe micr analyzer (EPMA), an X-ray diffractmeter (XRD) and a dry sand rubber wheel abrasin tester. 2. Experimental Prcedures 2.1 Specimen preparatin The substrate material fr the welding surface was prepared frm ASTM A36 steel plates with dimensins f 40 mm 40 mm 10 mm. In rder t btain hypeutectic, eutectic and hypereutectic (Cr,Fe) 23 C 6 structures, different amunts f graphite and chrmium pwder were mixed tgether. The cmpacts f different pwder mixtures were then prepared by a cnstant high pressure f 1500 psi ( kg/cm 2 ) in rder t frm ally filler with dimensins f 30 mm 25 mm 3 mm. The cmpnents f the three ally fillers used in this study are listed in Table 1. Bead-n-plate welding was accmplished by means f scillating GTAW prcess by using electric pwer supply alng with an aut-mechanized system in which the welding trch was mved back and frth at a cnstant speed abve the ally filler. The ranges f the welding cnditins used in this research study are listed in Table Characterizatin f cating The XRD specimens (12 mm 12 mm) were prepared frm the tp surface f the hardfacing allys. XRD with Cu Table 1 Ally filler cmpnents (mass%). Graphite Chrmium Ally filler A 5 95 Ally filler B 7 93 Ally filler C 9 91

2 Micrstructure and Wear Characteristics f Hypeutectic, Eutectic and Hypereutectic (Cr,Fe) 23 C 6 Carbides in Hardfacing Allys 2325 Table 2 Experimental cnditins fr GTAW surfacing. Parameter Value Electrde Type W-2%ThO Diameter 3.2 mm Angle 45 Vltage 15 V Current 200 A Prtective Gas Type Ar Flw 15 L/mm Welding speed Travel speed 30 mm/min Oscillating speed 250 mm/min Fig. 1 OM f the as-welded specimen. K radiatin ( nm) was used t analyze the cnstituent phases in the micrstructure. The micrstructures f the allys were examined by using bth OM and FE-SEM. The micrstructure investigatins were carried ut n the tp surface f the catings after plishing and etching. The etchant was cmpsed f 20 g f ammnium hydrgen fluride, 0.5 g f ptassium pyrsulfite and 100 ml f H 2 Oat80 C. An EPMA was utilized t analyze the chemical cmpsitin f the hardfacing allys. The bulk hardness f their tp surface was measured. Intensity, I/ cps : Cr-Fe : (Cr,Fe) 23 C 6 : (Cr,Fe) 7 C 3 Specimen C Specimen B Specimen A 2.3 Wear test Wear resistance experiments were cnducted using an ASTM G-65 dry sand rubber wheel abrasive testing apparatus. 11) The dry sand rubber wheel abrasin test is widely used t evaluate the lw-stress abrasive wear f materials, particularly fr the evaluatin f wear-resistant materials used in mining and agricultural machinery industries. 12,13) Prir t the wear test, the abrasive wear test specimens were grund by using 800 mesh emery paper. Subsequently, quartz sand with size ranging frm mm was used. The sand flw rate was apprximately 250 g/min, and the test lad during the wear test was chsen t be 13.5 kg. The speed f the dry sand rubber wheel in the abrasin test was set as 200 rpm. The weight lss was measured t within 1 mg. The wear specimens were thrughly cleaned and washed, and their wrn surfaces were bserved under an OM t characterize the surface mrphlgy and t study the pssible mechanism fr material remval. 3. Results and Discussin Surface mdificatin using GTAW and hardfacing ally cladding is a prcess in which chrmium and graphite ally fillers f desirable cmpsitins and a surface layer f the substrate material are simultaneusly melted and then rapidly slidified t frm a dense cating that is bnded t the base material. In this study, the substrate material used was carbn steel. Cnsequently, in additin t chrmium and carbn, the hardfacing layer als had irn, which resulted in the frmatin f Fe-Cr-C allys. N crack frmatin was Angle, 2θ / Fig. 2 X-ray spectrum f the hardfacing allys. bserved, and the melted surface shwed a smth rippled surface tpgraphy. Figure 1 shws the mrphlgy f the hardfacing layer. The thickness f the available catings with different carbn cmpsitins ranged frm 2 3 mm. Figure 2 shws the XRD spectrum f the surface f the hardfacing allys. Specimens A and B were bserved t cntain a slid slutin f Cr-Fe (JCPD ) with the bcc crystal structure ( phase) and (Cr,Fe) 23 C 6 (JCPD ) carbide with a cmplex fcc crystal structure. Specimen C was bserved t cntain three phases, i.e., (Cr,Fe) 23 C 6 and trace amunts f (Cr,Fe) 7 C 3 (JCPD ). Figures 3(a) and 4(a) shw the micrstructure f 5 mass% graphite. A primary Cr-Fe slid slutin matrix () and a fine eutectic structure (M 23 C 6 + ) are frmed. Its hypeutectic structure, eutectic is surrunded by the matrix. When the graphite cmpsitin reaches 7 mass%, the main carbide precipitate is M 23 C 6. The micrstructure in specimen B (7 mass%, graphite) is an (M 23 C 6 + ) eutectic structure, as shwn in Figs. 3(b) and 4(b). In the slidificatin prcess f GTAW, the Fe-Cr-C ally eutectics grw unifrmly in all directins until finally an equi-axed dendrite structure is btained. Cmparing Figs. 4(a) and 4(b), the eutectic grain is bserved t be carser than the hypeutectic grain, which is due t the different slidificatin rates fr each hardfacing ally.

3 2326 C. W. Ku, C. Fan, S. H. Wu and W. Wu Fig. 3 OM micrgraphs f hardfacing allying specimens: (a) specimen A, (b) specimen B and (c) specimen C. The micrstructures btained frm specimen C are presented in Figs. 3(c) and 4(c). It is evident that the primary phases, as shwn in Fig. 4(c), are carser than thse shwn in Figs. 4(a) and 4(b). A cmparisn amng Figs. 4(a), 4(b) and 4(c) shws that increasing the amunt f graphite pwder prmtes the frmatin f (Cr,Fe) 23 C 6 carbide. This type f structure is similar t the eutectic structure f a casting in high-carbn ferrchrmium. 14) The (Cr,Fe) 7 C 3 carbides are generally cvered with ther phases, which make it almst impssible t identify them with an OM. In this study, it was bserved clearly n a backscattered electrn image (BEI) f the FE-SEM by their different element cncentratin. This results in an apparent cntrast in the BEI image, as shwn in Fig. 5. The dark area represents (Cr,Fe) 7 C 3 and the gray area represents (Cr,Fe) 23 C 6. Fig. 4 FE-SEM micrgraphs f the hardfacing-ally-cladded specimens: (a) specimen A (hypeutectic structure), (b) specimen B (eutectic structure) and (c) specimen C (hypereutectic structure). The chemical cmpsitin f hardfacing allys was analyzed by EPMA and the results are listed in Table 3. The analyses were carried ut fr the surface f the specimen. The cmpsitins f hardfacing allys A, B and C are listed in Table 3. Based n this liquidus prjectin, the structures f specimens A, B and C crrespnd t Cr-Fe slid slutin (), eutectic ( +M 23 C 6 ) and M 23 C 6. Micrstructure f specimen A cnstructed frm the Cr-Fe slid slutin matrix and a fiber-like (Cr,Fe) 23 C 6 carbide (a hypeutectic structure), as shwn in Fig. 4(a). (Cr,Fe) 23 C 6 was the first t appear fllwing the cmmencement f the slidificatin f hardfacing ally C. Subsequently, an irregular and carse

4 Micrstructure and Wear Characteristics f Hypeutectic, Eutectic and Hypereutectic (Cr,Fe) 23 C 6 Carbides in Hardfacing Allys Wear Lss, W / mg/mm Specimen A Specimen B Specimen C Time, t /min Fig. 7 Wear lss f the hardfacing allys. Table 3 Fig. 5 BEI f hardfacing ally C. Chemical cmpsitins f hardfacing allys by EPMA. Hardfacing Cmpsitin (mass%) ally Cr Fe C Si Mn P S A B C Hardness, HRC Fig. 6 A B C Specimen Hardness f the hardfacing allys. eutectic structure was frmed arund the (Cr,Fe) 23 C 6 grain bundary, as shwn in Fig. 4(c). The hardness measurements perfrmed fr the hardfacing allys with different graphite cntents are shwn in Fig. 6. It is evident that the maximum hardness values were btained fr the surface f the hardfacing ally cladding f specimen C. This increase in hardness f the allyed surface f specimen C is assciated with the large amunts f (Cr,Fe) 23 C 6 and (Cr,Fe) 7 C 3, which pssess high carbn cntent as cmpared t the ther tw specimens. Figure 7 shws the wear lss f each specimen after being subjected t the dry sand rubber wheel abrasive test. It shws that the wear lss increases with the wear time. The wear resistance f hardfacing allys C is the wrst amng the wear resistance f all the allys, even if the appear f thick and big (Cr,Fe) 23 C 6 and trace (Cr,Fe) 7 C 3 carbide lead specimen C have the highest hardness (as shwn in Fig. 6). The fine (Cr,Fe) 23 C 6 structure has a greater effect n the wear resistance as cmpared t the carse (Cr,Fe) 23 C 6 structure. An examinatin f the wear scars indicated that the damage mrphlgies fr all the samples were different, as shwn in Fig. 8. The scar in specimen A was the finest (Fig. 8a) and the wear lss f the specimen was the least (Fig. 7). In additin, Figs. 3(c) and 4(c) shw that the hypereutectic structure cmprises a carse hypereutectic structure and a large amunt f (Cr,Fe) 23 C 6 carbide. This arrangement allws the hypereutectic structure t shed easily frm the catings because f the unsuitable cmbinatin f the massive carbide and the carse (Cr,Fe) 23 C 6 /Cr-Fe eutectic structure. Pits can be bserved (the shadw under the scars) in Figs. 8(b) and 8(c). Furthermre, the pits in specimen C are larger than thse in B. Mrever, specimen C has few scars but has hles prduced during the wear test. As a result, the wear lss becmes larger than the wear lsses f the hypeutectic and eutectic specimens, and a very rugh surface is frmed, as shwn in Fig. 8(c). Thus, it is assumed that the wear is due t the prgress f abrasive particles by preferentially abrading the matrix due t their abrasin, thereby gradually penetrating and crrding the matrix. This actin gradually increases the carbide cntent, and cnsequently the carbides must supprt the lad f the abrasive particles. By increasing the fineness f the carbides, the frce frm the abrasive particles is reduced, which imprves the abrasin resistance. Hwever, if the preferential wear f the matrix regins is excessive, then the carbides are n lnger supprted and they becme susceptible t spalling and fracture. In general, the tw cmpnents f wear resistant materials serve different functins. The hard particles impede the wear by grving r indenting the mineral particles, while the metal matrix prvides sufficient tughness. Bth prperties depend n the amunt, size and distributin f the hard particles as well as the hardness and fracture tughness f bth the cmpnents and the bnd between them. 4. Cnclusins In this study, the quantity f graphite was adjusted t

5 2328 C. W. Ku, C. Fan, S. H. Wu and W. Wu btain three types f structures: hypeutectic, eutectic and hypereutectic. These structures cnsisted f carbides with different mrphlgies and matrices. The abrasive wear results shw that the mrphlgy f (Cr,Fe) 23 C 6 carbides have a large effect n the abrasin resistance f the cating. The carser the (Cr,Fe) 23 C 6 carbides, the lwer is the wear resistance, even if the large (Cr,Fe) 23 C 6 carbides apprpriately cntribute t the hardness f the hardfacing layer. Furthermre, the wear behavir indicated that the abrasive wear resistance is nt nly simply related t the hardness f the depsit but als determined by the carbides and the matrix structure f the cating. Fine and highly-cncentrated carbide is better than a large amunt f carbide in an abrasive wear test because a thick and large structure causes large particles but nt scars t be dug ut during wearing. REFERENCES 1) L. Lu, H. Sda, and A. McLean: Mater. Sci. Eng. A 347 (2003) ) S. O. Yilmaz: Surf. Cat. Technl. 194 (2005) ) Y. Matsubara, N. Sasaguri, K. Shimizu, S. Yu, and K. Yu: Wear 250 (2001) ) A. F. Zhang, J. D. Xing, L. Fang, and J. Y. Su: Wear 257 (2004) ) H. Berns: Wear 254 (2003) 47. 6) J. D. Xing, Y. M. Ga, E. Z. Wang, and C. G. Ba: Wear 252 (2002) ) S. As, S. Gt, Y. Kmatsu, and W. Hartn: Wear 250 (2001) ) Q. Y. Hu, J. S. Ga, and F. Zhu: Surf. Cat. Technl 194 (2005) ) R. G. Wellman and J. R. Nichlls: Wear 242 (2000) ) M. Erglu and N. O zdemir: Surf. Cat. Technl 154 (2002) ) ASTM G65-94, Standard test methd fr measuring abrasin using the dry sand/rubber wheel apparatus, Annual Bk f ASTM Standards, Vl , (ASTM, Philadelphia, PA), pp ) X. Ma, R. Liu, and D. Y. Li: Wear 241 (2000) ) K. Elalem and D. Y. Li: Wear 250 (2001) ) A. Lesk and E. Navara: Mater. Charact. 36 (1996) Fig. 8 OMs f the hardfacing allys after being subject t wear fr 30 min: (a) hypeutectic structure, (b) eutectic structure and (c) hypereutectic structure.