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1.
The objective of this study is to investigate effects of heat treatment on wear resistance and fracture toughness in duo-cast materials composed of a high-chromium white cast iron and a low-chromium steel as a wear-resistant part and a ductile part, respectively. Different size, volume fraction, and distribution of M7C3 carbides were employed in the wear-resistant part by changing the amount of chromium, and the volume fraction of martensite in the austenitic matrix was varied by the heat treatment. In the alloys containing a small amount of chromium, an interdendritic structure of eutectic M7C3 carbides was formed, and led to the improvement of wear resistance and fracture toughness. After the heat treatment, the selective wear of the matrix and the cracking or spalled-off carbides were considerably reduced since the hardness difference between carbides and matrix decreased by the increase in the matrix hardness, thereby leading to the improvement of the wear resistance. However, the fracture toughness of the heat-treated alloys was lower than that of the as-cast alloys because the matrix containing a considerable amount of martensite did not effectively prevent the crack propagation.  相似文献   

2.
The changes in the microstructure and wear resistance of a powder metallurgical high-Cr white iron after the incorporation of TiC particles were studied in the present work. Various reinforcement volume fractions and sizes were used in order to examine their influence on the three-body abrasion wear resistance. The experiments were carried out at three different austenitizing temperatures. The most important observation after a microstructural examination was the increased amount of martensite in the composites subjected to identical heat treatment procedures with the unreinforced alloy. The austenite-to-martensite transformation in the composites increased with the TiC volume fraction and with the austenitizing temperature. This indicates that the two parameters have a key role in the transformation mechanism, which seems to be mechanically induced. The increasing of martensitic transformation with the TiC content in the composites enhanced continuously the supporting ability of the iron alloy matrix to the TiC particles, which in turn increased the wear resistance of the composites. The abrasion wear resistance increased with the TiC volume fraction until the onset of spalling. However, in composites containing coarse reinforcements, spalling occurred earlier in the wear process. This decreased wear resistance significantly because spalled TiC particles contributed additionally to wear.  相似文献   

3.
The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M2C carbides formed during the dissolution of the cementite (M3C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M2C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M2C carbide, while Co increased the nucleation rate of M2C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M2C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.  相似文献   

4.
5.
The effect of W on dislocation recovery and precipitation behavior was investigated for martensitic 9Cr-(0,l,2,4)W-0.1C (wt pct) steels after quenching, tempering, and subsequent prolonged aging. The steels were low induced-radioactivation martensitic steels for fusion reactor structures, intended as a possible replacement for conventional (7 to 12)Cr-Mo steels. During tempering after quenching, homogeneous precipitation of fine W2C occurred in martensite, causing secondary hardening between 673 and 823 K. The softening above the secondary hardening temperature shifted to higher temperatures with increasing W concentration, which was correlated with the decrease in self-diffusion rates with increasing W concentration. Carbides M23C6 and M7C3 were precipitated in the 9Cr steel without W after high-temperature tempering at 1023 K. With increasing W concentration, M7C3 was replaced by M23C6, and M6C formed in addition to M23C6. During subsequent aging at temperatures between 823 and 973 K after tempering, the recovery of dislocations, the agglomeration of carbides, and the growth of martensite lath subgrains occurred. Intermetallic Fe2W Laves also precipitated in the δ-ferrite grains of the 9Cr-4W steel. The effect of W on dislocation recovery and precipitation behavior is discussed in detail.  相似文献   

6.
The effect of carbide morphology and matrix structure on abrasion resistance of cast alloyed steel with 2.57% C, 16.2% Cr and 0.78% Mo was studied in the as‐cast and heat treated conditions. Samples were austenitized at three different temperatures of 980, 1050 and 1250 °C for 15 minutes and followed by tempering at 540 °C for 3 hours. The austenitizing temperature of 980 °C revealed fully martensitic structure with little amount of retained austenite, while at 1050 °C the matrix was austenitic with massive amount of coarse secondary carbides. The austenitic matrix with very fine secondary carbides was developed at 1250 °C. The maximum abrasion resistance was obtained at 1050 °C due to the highest structure hardness and existence of both eutectic and secondary carbides in larger size than the formed groove by the abrasive particles during the wear test. On the other hand, the as‐cast pearlitic structure showed high wear rate by an applied load of up to 0.2 bar, followed by very rapid increase in wear rate with higher applied loads. It could be considered that the austenitizing temperature of 1050 °C showed better combination of abrasion resistance and toughness in comparison with other heat treatment cycles.  相似文献   

7.
The fractures of three model alloys, imitating by their chemical composition the matrixes of the quenched high-speed steels of various Mo: W relations were analyzed. According to the measurements of the stress intensity factor KIc and the differences in the precipitation processes of carbides it was found out that the higher fracture toughness of the matrix of the molybdenum high-speed steels than on the tungsten ones is the results of the differences in the kinetics of precipitation from the martensite matrix of these steels during tempering. After tempering at 250 and 650°C the percentage of the intergranular fracture increases with the increase of the relation of Mo to W in the model alloys of the high-speed steel matrix. This is probably the result of higher precipitation rate of the M3C carbide (at 250°C) and the MC and M6C carbides (at 650°C) in the privileged regions along the grain boundaries. The change of the character of the model alloy fractures after tempering at 450°C from the completely transgranular one in the tungsten alloy to the nearly completely intergranular one in the molybdenum alloy indicates that the coherent precipitation processes responsible for the secondary hardness effect in the tungsten matrix begin at a lower temperature than in the molybdenum matrix. After tempering for the maximum secondary hardness the matrix fractures of the high-speed steels reveal a transgranular character regardless the relation of Mo to W. The higher fracture toughness of the Mo matrix can be the result of the start of the coherent precipitation processes at a higher temperature and their intensity which can, respectively, influence the size of these precipitations, their shape and the degree of dispersion. The transgranular character of the fractures of the S 6-5-2 type high-speed steel in the whole range tempering temperatures results from the presence of the undissolved carbides which while cracking in the region of stress concentration can constitute flaws of critical size which form the path of easy cracking through the grains. The transgranular cracking of the matrix of the real high-speed steels does not change the adventageous influence of molybdenum upon their fracture toughness. On the other hand, the carbides, undissolved during austenitizing, whose size distribution in the molybdenum steels from the point of view of cracking mechanics seems to be unsatisfactory, influence significantly the fracture toughness of these steels.  相似文献   

8.
Tempered martensitic structure is the service condition of T91 ferritic steel after adopting the austenitizing followed by tempering. Needle‐like M3C particles are precipitated during air cooling after austenization, while the precipitation of M3C is suppressed during the water cooling. The effect of existence of M3C on the precipitation behaviors of M23C6 during the early stage of tempering, as nucleation site, number density and size distribution, was investigated by means of TEM observation. The TEM results indicate that, upon the same tempering time, the size of M23C6 is smaller and its number density is higher in the sample pre‐existing M3C than in the sample without M3C. This can be explained that existence of M3C results in more M23C6 precipitates forming inside of grain, where a relatively low self‐diffusion coefficient of alloy element leads to M23C6 hardly coarsening. However, with the prolongation of tempering time, this effect becomes weaken. Microhardness results indicate that the existence of M3C phase results in the increase of hardness after tempering due to the precipitation of finer and denser M23C6 particles.  相似文献   

9.
The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated. Complementing an earlier study[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy. With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to vir-tually no change in wear performance and substantial falls in hardness. Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature. Tempering treatments in the range 400 °C to 600 °C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hard-ness and wear performance, but negligible changes in fracture toughness. Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer.  相似文献   

10.
J. Ju  D.-M Fu  S.-Z. Wei  P. Sang  Z.-W. Wu 《钢铁冶炼》2018,45(2):176-186
The effects of chromium and vanadium additions on the microstructure, hardness and wear resistance of high-vanadium alloy steel (containing 5–10 wt-% V and 2–10 wt-% Cr) were studied by means of optical microscopy, scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS), Vickers hardness and Rockwell-hardness tester & M-200 ring block wear tester. Researching results showed that the solidification structure of high-vanadium wear-resistant alloy steel was mainly consisted α-Fe (martensite), vanadium carbide (VC), M3C and M7C3. Vanadium is mainly distributed over VC, and certain amount of vanadium exists in the matrix and M7C3 type eutectic carbide. Chromium is mainly distributed over the M7C3, and the matrix also contains a small quantity of chromium. It is found that the content of VC increases with the increase of vanadium content when carbon and chromium contents are constant. The change of micro- and macro-hardness was not obvious with the increase of vanadium content. The content of M7C3 type eutectic carbides increases gradually with the increase of chromium content when carbon and vanadium contents are constant. The micro- and macro-hardness increases with the increase of chromium content. The increase of vanadium content brings to the increase of wear resistance of alloy steel when carbon and chromium contents are constant. The change of chromium content had no obvious effect on wear resistance of high-vanadium alloy steel when carbon and vanadium contents. The increase of vanadium content brings to the increase of wear resistance of alloy steel when carbon and chromium contents are constant. The wear resistance of as-cast high-vanadium alloy steel is the best when the content of vanadium and chromium is 10 wt-% and 5 wt-% respectively.  相似文献   

11.
The fatigue crack growth rates,da/dN, and the fracture toughness, KIc have been measured in two high-carbon martensitic stainless steels, 440C and BG42. Variations in the retained austenite contents were achieved by using combinations of austenitizing temperatures, refrigeration cycles, and tempering temperatures. In nonrefrigerated 440C tempered at 150 °C, about 10 vol pct retained austenite was transformed to martensite at the fracture surfaces duringK Ic testing, and this strain-induced transformation contributed significantly to the fracture toughness. The strain-induced transformation was progressively less as the tempering temperature was raised to 450 °C, and at the secondary hardening peak, 500 °C, strain-induced transformation was not observed. In nonrefrigerated 440C austenitized at 1065 °C,K Ic had a peak value of 30 MPa m1/2 on tempering at 150 °C and a minimum of 18 MPa m1/2 on tempering at 500 °C. Refrigerated 440C retained about 5 pct austenite, and did not exhibit strain-induced transformation at the fracture surfaces for any tempering temperature. TheK Ic values for corresponding tempering temperatures up to the secondary peak in refrigerated steels were consistently lower than in nonrefrigerated steels. All of the BG42 specimens were refrigerated and double or quadruple tempered in the secondary hardening region; theK Ic values were 16 to 18 MPa m1/2 at the secondary peak. Tempered martensite embrittlement (TME) was observed in both refrigerated and nonrefrigerated 440C, and it was shown that austenite transformation does not play a role in the TME mechanism in this steel. Fatigue crack propagation rates in 440C in the power law regime were the same for refrigerated and nonrefrigerated steels and were relatively insensitive to tempering temperatures up to 500 °C. Above the secondary peak, however, the fatigue crack growth rates exhibited consistently lower values, and this was a consequence of the tempering of the martensite and the lower hardness. Nonrefrigerated steels showed slightly higher threshold values, ΔKth, and this was ascribed to the development of compressive residual stresses and increased surface roughening in steels which exhibit a strain-induced martensitic transformation.  相似文献   

12.
The carbides in M-50 high speed tool steel were studied in detail. The dissolution of carbides as a function of austenitizing temperature, and their precipitation as a function of tempering temperature were characterized by X-ray diffraction and microchemical analysis. The carbides in the annealed steel are M23C6, M6C, M2C, and MC. Upon austenitizing, with increasing temperatures, the carbides dissolve in the order: M23C6, metastable M2C, M6C, and MC. The residual carbides in the heat treated steel are MC and stable M2C. The solvus temperatures of M23C6 and M6C were determined. Upon tempering the hardened steel, with increasing tempering temperatures, carbides precipitate in the order: M23C6, metastable M2C, MC, and M6C. It is shown that the composition of the precipitated metastable M2C is different from that of the residual stable M2C and it varies with the tempering temperature.  相似文献   

13.
A series of heat treatments was employed to vary the microstructure of four commercially important alloy white cast irons, the wear resistance of which was then assessed by the ASTM jaw-crusher gouging abrasion test. Compared with the as-cast condition, standard austenitizing treatments produced a substantial increase in hardness, a marked decrease in the retained aus-tenite content in the matrix, and, in general, a significant improvement in gouging abrasion resistance. The gouging abrasion resistance tended to decline with increasing austenitizing tem-perature, although the changes in hardness and retained austenite content varied, depending on alloy composition. Subcritical heat treatment at 500 ° following hardening reduced the retained austenite content to values less than 10 pct, and in three of the alloys it caused a significant fall in both hardness and gouging abrasion resistance. The net result of the heat treatments was the development of optimal gouging abrasion resistance at intermediate levels of retained aus-tenite. The differing responses of the alloys to both high-temperature austenitizing treatments and to subcritical heat treatments at 500 ° were related to the effects of the differing carbon and alloying-element concentrations on changes in theM s temperature and secondary carbide precipitation.  相似文献   

14.

Fast removal of soft phases (e.g., pearlite and ferrite) in the iron matrix limits the wear life of high-Cr white irons. To address this shortcoming, the authors successfully produced fine networks of M6C carbide in a high-Cr white iron through extensive thermodynamic calculations. Fishbone-like networks of M6C carbides were observed with an optical microscope. It was experimentally determined that such carbide networks protected the soft matrix and increased the overall hardness. Additionally, electron backscattered diffraction was conducted, which showed that the alloy contained phases of M7C3, M6C, ferrite, and retained austenite. Solidification sequence was determined by correlating the thermodynamic equilibrium calculation results with the size and distribution of each phase. A dry sand/rubber wheel apparatus following ASTM standard G65 Procedure A was utilized to assess the abrasive wear performance of the developed alloy. Results showed that the volume loss of the developed material was 25 pct less than that of conventional high-Cr white irons. Wear scars were investigated using a scanning electron microscope, and the improved wear resistance was attributed to the “buffer” effect and plastic deformation of the introduced M6C carbide networks.

  相似文献   

15.
Correlation of microstructure with hardness and wear resistance of (CrB,MoB)/carbon steel surface composites fabricated by high-energy electron beam irradiation was investigated in this study. Three kinds of powder mixtures, i.e., 50CrB-50MgF2(flux), 50MoB-50MgF2, and 25CrB-25MoB-50MgF2 (wt pct), were placed on a plain carbon steel substrate, which was then irradiated with the electron beam. In the specimens fabricated with flux powders, the surface composite layer of 0.8 to 1.3 mm in thickness was successfully formed without defects, and contained a large amount (up to 48 vol pct) of Cr1.65Fe0.35B0.9 or Mo2FeB2 in the martensitic matrix. The hardness and wear resistance of the surface composite layer were directly influenced by the hard borides, and thus were about 3 to 7 times greater than those of the steel substrate. Particularly, in the surface composite fabricated with CrB and MoB powders, the hardness of eutectic solidification cells and martensitic matrix was very high, and borides formed a network structure along cells, thereby leading to the best hardness and wear resistance. These findings suggested that the high-energy electron beam irradiation was useful for the development of surface composites with improved hardness and wear resistance.  相似文献   

16.
The changes in matrix structure that occur during tempering of an Fe-0.2C martensite at 400° to 700°C have been investigated. Light and electron metallographic observations show that when tempered, the fine martensitic lath structure coarsens while retaining the elongated packet-lath morphology. The as-quenched hardness 504 Khn and total grain boundary area per unit volume 50,800 cm−1 decrease abruptly at the higher tempering temperatures and in seconds reach relatively stable values that decrease slowly with time. The decrease in low angle boundaries accounts for most of the initial grain boundary area change, while the large angle boundary component of total boundary area decreases gradually with tempering time. Recovery processes are responsible for the initial changes in matrix structure, and carbide boundary pinning suppresses recrystallization until grain growth dominates in the later stages of tempering.  相似文献   

17.
The correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC + SiC powders and CaF2 flux were placed on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2 to 2.1 mm in thickness were formed without defects and contained a large amount (up to 66 vol pct) of precipitates such as TiC and Ti5Si3 in the martensitic matrix. This microstructural modification, including the formation of hard precipitates and a hardened matrix in the surface composite layer, improved the hardness and wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate, because 66 vol pct of TiC and Ti5Si3 was precipitated homogeneously in the hardened martensitic matrix. These findings suggested that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved hardness and wear properties.  相似文献   

18.
A new type of high boron-high speed steel (HB-HSS) with different boron content was selected for oil quenching at 1050 °C, and different temperature of tempering treatment was chosen. By using optical microscopy, scanning electron microscopy, X-ray diffraction, Rockwell hardness tester, red hard treatment and wear test, the effects of heat treatment on microstructure and properties of HB-HSS were investigated. The experimental results indicate that the quenching microstructure of HB-HSS consists of α-Fe, M2(B, C), M7(B, C)3 and a few of M23(C, B)6. When the tempering temperature is lower than 500 °C, the shape of carboborides will change from discontinuous sheet to continuous net, and the uniformity in microstructure is improved, and the hardness is not changed during the process. When the tempering temperature is higher than 500 °C, the continuous net of M2(B, C) is recovered. When the tempering temperature is higher than 600 °C, the microstructure of HB-HSS get thickened because of overheating, and the hardness get significantly reduced. With the increase of tempering temperature, the weight loss of the sample is decreased, and the wear resistance of the sample is increased. When tempering temperature exceeds 500 °C, the weight loss of the sample has an obvious increase and its wear resistance decreases. The wear resistance of the sample decreases after the red-hardness treatment. The wear loss is about 8.4 mg when the boron content is 2.0% and the tempering temperature is 500 °C, which is the best of test samples.  相似文献   

19.
The structure and mechanical properties of 35Kh12G3MVFDR steel are investigated. After normalization or quenching, the steel contains up to 35 vol % austenite and may be assigned to the martensitic–austenitic class. On heat treatment—tempering, isothermal holding, or isothermal quenching—the austenite is converted to martensite within 2 h. The martensite in 35Kh12G3MVFDR steel is more thermally stable: the first signs of its conversion to sorbitic structure are observed after 25-h isothermal quenching at 640°C, and its complete decomposition requires 50 h. The breakdown of martensite is accompanied by decrease in the high-temperature strength and hardness. Aging of the quenched and tempered 35Kh12G3MVFDR steel at 670–720°C lowers the hardness from 61–65 HRA to 55–60 HRA after 1600–3200 h and the yield point at 20°C from 1350 MPa to 750–850 MPa. Likewise, the yield point at 720°C declines from 310 MPa to 160–230 MPa after 600 h and then stops. The state of the martensitic structure of 35Kh12G3MVFDR steel determines its creep resistance at 700°C. For example, the martensite remains in the steel structure after relatively brief isothermal quenching (up to 24 h at 640°C), and consequently the creep limit σ700°C 0.1%/h is no lower than after simple quenching with subsequent high tempering: 86.2 ± 9.4 MPa and 89.3 ± 8.8 MPa, respectively. At the same time, in response to the decomposition of martensitic structure as a result of prolonged aging (1600 h at 670°C), σ700°C 0.1%/h declines to 63.9 ± 7.1 MPa. In contrast to martensite, the austenite in 35Kh12G3MVFDR steel is thermally unstable and is converted to martensite after only 1–2 h of heating, depending on the temperature.  相似文献   

20.
The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M7C3 carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M2C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.  相似文献   

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