Effects of alloying elements on microstructure,hardness, and fracture toughness of centrifugally cast high-speed steel rolls

A study was made of the effects of alloying elements on the microstructure, hardness, and fracture toughness of centrifugally cast high-speed steel (HSS) rolls. Particular emphasis was placed on the role of hard carbides located along solidification cell boundaries and the type of the tempered martensitic matrix. Microstructural observation, X-ray diffraction analysis, hardness and fracture-toughness measurements, and fractographic observations were conducted on the rolls. The constitution and morphology of carbides observed within the intercellular boundaries varied depending upon the predominant alloying elements that comprised them. These massive carbide formations strongly influenced the bulk material hardness and fracture toughness due to their high hardness. The effects of alloying elements were analyzed on the basis of the liquidus-surface diagram which and indicated that the proper contents of the carbon equivalent (CE), tungsten equivalent, and vanadium were 1.9 to 2.0, 10 to 11, and 5 to 6 pct, respectively. The roll material, containing a small amount of intercellular carbides and lath-type tempered martensitic matrix, had excellent fracture toughness, since carbides were well spaced. Therefore, it was suggested that the optimization of alloying elements was required to achieve the homogeneous distribution of carbides.     

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

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.     

Correlation of microstructure and thermal-fatigue properties of centrifugally cast high-speed steel rolls

This is a study of thermal-fatigue properties in centrifugally cast high-speed steel (HSS) rolls. The thermal-fatigue mechanism was investigated, with special focus on the roll microstructure and the increase in tensile stress which led the specimens to fracture when it reached the tensile strength. The thermal-fatigue test results indicated that the thermal-fatigue life decreased with increasing maximum temperature of the thermal-fatigue cycle. The results were then interpreted based on the amount of carbides and the cyclic-softening phenomenon associated with the exposed time to elevated temperatures. The coarse intercellular carbides on the specimen surface acted as fatigue-crack initiation sites as they cleaved at a low stress level to form cracks. The roll having the lowest matrix hardness and the smallest amount of intercellular carbides, thus, showed better thermal fatigue properties than the other rolls. For the improvement of the thermal-fatigue properties of the rolls, this study suggests a homogeneous distribution of carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing the roll compositions.     

Correlation of microstructure and thermal fatigue property of three work rolls

This is a study of the thermal fatigue property in three centrifugally cast work rolls, i.e., a nickel-grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a high-speed steel roll (HSS roll). The thermal fatigue mechanism was investigated with a focus on the roll microstructure and the increase in tensile stress which led the specimen to fracture when it reached the tensile strength. The thermal fatigue test results indicated that the thermal fatigue property was best in the HSS roll, followed by the Hi-Cr roll and the Ni-grain roll, respectively, and that the thermal fatigue life of each roll decreased with the increase of the mean temperature or of the temperature range of the thermal fatigue cycle. The results were then interpreted based on the amount of primary carbides and the cyclic softening phenomenon associated with the exposed time to elevated temperatures. The coarse primary carbides on the specimen surface acted as fatigue crack initiation sites, as they cleaved at a low stress level to form cracks. The HSS roll, having the highest tensile strength and the smallest amount of primary carbides, thus showed better thermal fatigue property than the other rolls. For the improvement of the thermal fatigue property of the rolls, this study suggests a homogeneous distribution of primary carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing of the roll-casting process.     

Correlation of microstructure with wear and fracture properties of two-layered VC/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with the hardness, wear resistance, and fracture toughness of two-layered VC/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. A mixture of VC powders and CaF₂ flux was deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these powder mixtures to fabricate an one-layered surface composite. A two-layered surface composite was fabricated by irradiating an electron-beam again onto the powder mixture deposited on the one-layered surface composite. The composite layers of 1.2 to 1.5 mm in thickness were homogeneously formed without defects and contained a large amount (25 to 40 vol pct) of carbides in the martensitic or β-Ti matrix. This microstructural modification, including the formation of hard carbides and hardened matrix, improved the hardness and wear resistance. Particularly in the two-layered surface composite containing more carbides, the wear resistance was greatly enhanced to a level 7 times higher than that of the Ti-6Al-4V substrate. In-situ observation of the fracture process showed that microcracks were initiated at carbides and propagated along these microcracked carbides and that shear bands were formed in the matrix between these microcracks. In the two-layered surface composite, numerous microcracks were initiated at many carbides and then rapidly propagated along them, thereby lowering the fracture toughness.     

The effect of high-energy electron-beam irradiation on microstructural modification of a high-speed steel roll

The purpose of this study is to investigate the microstructural modification in a high-speed steel (HSS) roll irradiated with an accelerated high-energy electron beam. The HSS roll samples were irradiated at the beam travel speeds of 2.5 to 25 mm/s using an electron accelerator (1.4 MeV). The microstructure was examined with a scanning electron microscope (SEM) capable ofin situ fracture testing and simultaneous measurement of the apparent fracture toughness. Irradiation changed the matrix phase from tempered martensite to a mixture of retained austenite and martensite. Coarse primary carbides were partially or completely dissolved, depending on the heat input. Irradiation greatly improved the fracture properties because of the presence of retained austenite, which could retard crack propagation, although hardness was decreased. Occasional interior quench cracks were found in the heat-affected region. Appropriate processing methods, such as pre- or postirradiation, were suggested. A heat transfer analysis of the irradiated surface layer was also carried out to elucidate the influence of the irradiation parameters on the microstructure.     

Effects of carbon and chromium additions on the wear resistance and surface roughness of cast high-speed steel rolls

The effects of carbon and chromium additions on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by a centrifugal casting method were investigated in this study. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The wear-test results revealed that the wear resistance increased with increasing carbon content because of the increased total carbide fraction. In the HSS roll containing a smaller amount of chromium, the wear resistance was improved because it contained a number of very hard MC carbides. However, it showed a very rough roll surface because of the preferential removal of the matrix and the sticking of the rolled material to the roll surface during the wear process. Rolls containing a higher chromium content had very low surface roughness, because of the formation of a larger amount of M₇C₃ carbides of slightly lower hardness and homogeneous wear of the matrix and carbides. In order to improve wear resistance with consideration of surface roughness, the increase in the chromium content, i.e., the increase in the amount of M₇C₃ carbides, which can result in a low surface roughness, was suggested.     

淬火温度对添加B_4C的ASP30粉末冶金高速钢组织及力学性能的影响

采用粉末冶金方法制备添加B_4C的全致密ASP30高速钢,样品在1 040℃到1 200℃范围内淬火,并且经过560℃三次回火,研究淬火温度对其力学性能及显微组织的影响。采用扫描电子显微镜、洛氏硬度计和材料力学性能测试机研究高速钢的组织和力学性能。结果表明:添加质量分数为0.025%B_4C的ASP30粉末冶金高速钢在1 160℃下烧结2 h后会形成月牙形液相碳化物,从而获得全致密的烧结组织。随淬火温度升高,显微组织中碳化物的数量明显减少,基体中合金元素固溶含量提高,基体晶粒长大,断口形貌呈准解理断裂但断口平整度下降。随淬火温度升高,钢的硬度提高,最高值达到69 HRC。抗弯强度、断裂韧性均下降,抗弯强度最高值达4 357MPa,断裂韧性最高值为48.6 MPa/m1/2。冲击韧性先升高后下降,在1 080℃最高为18.85 J/cm2。     

The effects of titanium additions on AF1410 ultra-high-strength steel

The mechanical properties and microstructure of two heats of AF1410 steel were compared. The first heat, heat 811, contained a titanium addition of 0.02 wt pct, while the second heat, heat 91, contained no titanium, manganese, or other strong sulfide formers. The sulfur in heat 811 was gettered as titanium carbosulfide, while in heat 91 the sulfides were chromium sulfide. The toughness of heat 811 was found to be much enhanced compared to heat 91, with Charpy impact energies of 176 J and 79 J and K_IC fracture toughness values of 235 MPa.m^1/2 and 170 MPa.m^1/2, respectively. This significant difference in fracture toughness is attributed to the fact that titanium carbosulfide particles are more resistant to void nucleation than the chromium sulfide particles, which appear to nucleate voids at the onset of plastic strain. In addition to altering the sulfide particle type, the titanium addition also results in the presence of undissolved MC carbides in the titanium-modified steel in addition to the M₂C carbides found in heat 91. These carbides act as grain growth inhibitors, resulting in a finer prior austenite grain size and martensite packet size in heat 811.     

Effects of heat treatment on wear resistance and fracture toughness of duo-cast materials composed of high-chromium white cast iron and low-chromium steel

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.     

Multiphase Microstructure Optimization to Enhance the Mechanical Properties of AISI M35 High-Speed Steel

It is crucial to conduct in-depth research on the cryogenic-treatment mechanism to promote the standardization and industrialization of cryogenic treatment in the high-speed steel (HSS) industry. In this study, the microstructure and mechanical properties (microhardness and impact toughness) of AISI M35 HSS after deep-cryogenic treatment (DCT) and conventional heat treatment (CHT) are investigated, and the microstructural characteristics at different stages of CHT and cryogenic treatment are studied. It is indicated in the results that DCT of the steel leads to the formation of fresh martensite from residual austenite, as well as the introduction of more dislocations due to plastic deformation. In addition, the deep-cryogenic-treated specimen that is tempered shows increased numbers of martensite blocks and secondary carbide precipitation. The carbides in the steel are mainly V-rich (MC), W–Mo-rich (M₆C), and Cr-rich (M₂₃C₆). The hardness of the deep-cryogenic-treated samples increases by approximately 50 HV₁ because of the transformation of residual austenite and dislocation strengthening. Furthermore, specimens that are both deep-cryogenic treated and tempered exhibit a 30% increase in impact toughness and a more uniform distribution in hardness, likely due to the more homogeneous precipitation of secondary carbides and refinement of martensite.     

On the Microstructure and Wear Behavior of Fe–xCr–4Mn–3C Hardfacing Alloys

The present paper describes an investigation aimed at evaluating the microstructural and dry sliding adhesive wear characteristics of Fe–xCr–4Mn–3C hardfacing alloys applied through shielded metal arc welding. The effect of chromium addition on the microstructure of hardfacings was carried out by using optical microscope, field emission scanning electron microscope and X-ray mapping. Dry sliding wear tests were performed on a pin-on-disc wear tribometer. From the experimental results, it was observed that the primary carbides were refined and increased with the increase of chromium content. The morphology of carbides revealed that the primary carbides were rod shaped. The increased chromium content was also found to be beneficial to enhance hardness and wear resistance of hardfacings. The correlation between hardness and wear resistance exhibited the reliability of hardness as an indicator of the wear performance of hardfacings.     

Effects of Alloying Elements on Microstructure,Hardness, Wear Resistance,and Surface Roughness of Centrifugally Cast High-Speed Steel Rolls

A study was made of the effects of carbon, tungsten, molybdenum, and vanadium on the wear resistance and surface roughness of five high-speed steel (HSS) rolls manufactured by the centrifugal casting method. High-temperature wear tests were conducted on these rolls to experimentally simulate the wear process during hot rolling. The HSS rolls contained a large amount (up to 25 vol pct) of carbides, such as MC, M₂C, and M₇C₃ carbides formed in the tempered martensite matrix. The matrix consisted mainly of tempered lath martensite when the carbon content in the matrix was small, and contained a considerable amount of tempered plate martensite when the carbon content increased. The high-temperature wear test results indicated that the wear resistance and surface roughness of the rolls were enhanced when the amount of hard MC carbides formed inside solidification cells increased and their distribution was homogeneous. The best wear resistance and surface roughness were obtained from a roll in which a large amount of MC carbides were homogeneously distributed in the tempered lath martensite matrix. The appropriate contents of the carbon equivalent, tungsten equivalent, and vanadium were 2.0 to 2.3, 9 to 10, and 5 to 6 pct, respectively.     

Correlation of microstructure with the wear resistance and fracture toughness of hardfacing alloys reinforced with complex carbides

A correlation was made of the microstructure, wear resistance, and fracture toughness of hardfacing alloys reinforced with complex carbides. The hardfacing alloys were deposited twice on a low-carbon steel substrate by a submerged arc welding (SAW) method. In order to investigate the effect of complex carbides, different fractions of complex carbide powders included inside hardfacing electrodes were employed. Microstructural analysis of the hardfaced layer showed that cuboidal carbides, in which a TiC carbide core was encircled by a WC carbide, and rod-type carbides, in which W and Ti were mixed, were homogeneously distributed in the bainitic matrix. In the surface layer hardfaced with FeWTiC powders, more complex carbides were formed, because of the efficient melting and solidification during hardfacing, than in the case of hardfacing with WTiC powders. As the volume fraction of complex carbides, particularly that of cuboidal carbides, increased, the hardness and wear resistance increased. In-situ observation of the fracture process showed that microcracks were initiated at complex carbides and that shear bands were formed between them, leading to ductile fracture. The hardness, wear resistance, and fracture toughness of the hardfacing alloys reinforced with complex carbides were improved in comparison with high-chromium white-iron hardfacing alloys, because of the homogeneous distribution of hard and fine complex carbides in the bainitic matrix.     

Analysis and prevention of microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel

This study was concerned with the effects of microstructural parameters on the microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the microcracked regions showed that microcracks were formed mainly along tortoise-shell-shaped depressions and that their number and size were considerably reduced when strip casting was done right after a shot-blasting or pickling treatment of the casting roll surface. This microcracking phenomenon was closely related to the formation of a black oxide layer, which was mainly composed of manganese-rich oxides, on the roll surface. The black oxide layer acted as a barrier of thermal transfer between the rolls and melt, led to an increased gas gap and inhomogeneous solidification of cast strips, and, thus, played a role in forming both tortoise-shell—shaped depressions and microcracks on the strip surface. The installation of brush rolls behind the casting rolls was suggested as a method to prevent microcracks, because the brush rolls could continuously scrape off the black oxide layer affixed on the roll surface during strip casting.     

Composition, microstructure, hardness, and wear properties of high-speed steel rolls

The effects of alloying elements on the microstructural factors, hardness, and wear properties of four high-speed steel (HSS) rolls fabricated by centrifugal casting were investigated. A hot-rolling simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the HSS roll containing a larger amount of vanadium showed the best wear resistance because it contained a number of hard MC-type carbides. However, it showed a very rough roll surface because of cracking along cell boundaries, the preferential removal of the matrix, and the sticking of the rolled material onto the roll surface during the wear process, thereby leading to an increase in the friction coefficient and rolling force. In order to improve wear resistance with consideration to surface roughness, it is suggested that a reduction in the vanadium content, an increase in solid-solution hardening by adding alloying elements, an increase in secondary hardening by precipitation of fine carbides in the matrix, and formation of refined prior austenite grains by preaustenitization treatment be employed to strengthen the matrix, which can hold hard carbides in it.     

变质处理高碳高速钢导板的研究

导板是线材轧机的关键备件，要求有优良的耐磨性，抗断裂性，抗粘钢性和热冲击抗力，常用高镍镍钢导板和高铬铸铁导板满足不了上述要求，使用寿命短，更换频繁，降低了轧机作业率，增大了工人劳动强度，高速钢具有硬度高，红硬性好和耐磨性好等特点，但铸造高速钢脆性大，通过调整高速钢成分和采用RE－Mg-Ti复合变质处理，可改变共晶碳化物的形态和分布，使高速钢冲击韧性大幅度提高，变质处理高速钢导板使用中不粘钢，不断裂，耐磨性明显优于普通导板。     

喷射成形含铌M3型高速钢的组织和耐磨性

采用喷射成形技术制备了M3型高速钢和以Nb替代V的M3型高速钢.利用扫描电镜、X射线衍射、差示扫描量热仪和金相显微镜研究了Nb对M3型高速钢组织的影响.喷射成形能有效消除宏观偏析,细化组织.以Nb代V,提高了MC型碳化物开始析出温度,大量MC相先于共晶反应析出,呈独立的近球形分布于晶界,同时其尺寸减小.由于消耗大量C,抑制了共晶反应,M₂C片层数量减少且厚度变薄,其在热变形过程中更易于分解,进一步增加了组织均匀性.低温低载荷时含铌的M3型高速钢抗磨损性能显著优于M3高速钢,温度升高到500℃时磨损机制逐渐以氧化磨损为主,两合金的抗磨损性能差距减小,主要原因是大量呈弥散球形分布的含铌MC型碳化物能有效提高高速钢的磨粒磨损抗性,而其对抗氧化性能并无明显作用.      

Microstructure evolution of TiC cermets with ferritic AISI 430L steel binder

From the outlook of healthcare, economic importance and supply risk, utilisation of raw materials like tungsten, cobalt and nickel should be reduced or replaced with other metals. Nontoxic titanium carbide and iron are the top-of-the-line solution for displacing these materials. Our focus was on conventionally fabricated titanium carbide-based cermets with a chromium ferritic steel binder. To study microstructural evolution, specimens were sintered at different temperatures (600–1500°C). We used a scanning electron microscopy, X-ray diffraction and differential scanning calorimetry to analyse the microstructure and phase formation of the cermets. Our results showed that during the solid and liquid phase sintering of the TiC–FeCr cermet, chromium ferrous complex carbides M₇C₃ are formed and as a result, chromium content in the binder phase is decreased. Alloying TiC–FeCr cermets with strong carbide formers improves the structural homogeneity of the cermets. Also, mechanical characteristics (hardness, fracture toughness) were evaluated.     

Influence of rare earth nanoparticles and inoculants on performance and microstructure of high chromium cast iron

The high chromium cast irons (HCCIs) with rare earth (RE) nanoparticles or inoculants were fabricated in the casting process. The phase compositions and microstructure were analyzed by X-ray diffraction (XRD) and optical microscopy (OM), respectively. The hardness and impact toughness were tested by Rockwel-hardmeter and impacting test enginery. And then, the morphology of fracture was researched by scanning electron microscopy (SEM). The results demonstrated that the phase compositions of HCCIs with addition of RE nanoparticles or inoculants which were M7C3 carbides + α-Fe did not change obviously. However, the prime M7C3 carbides morphology had great changes with the increase of RE nanoparticles, which changed from long lath to granular or island shape. When the content of RE nanoparticles was 0.4 wt.%, the microstructure of high chromium cast iron was refined greatly. The microstructure of carbides was coarser when the addition of RE nanoparticles was higher than 0.4 wt.%. The hardness and impact toughness of HCCIs were improved by addition of RE nanoparticles or inoculants. The impact toughness of HCCIs was increased 36.4% with RE nanoparticles of 0.4 wt.%, but the hardness changed slightly. In addition, the adding of RE nanoparticles or inoculants could reduce the degree of the brittle fracture. Fracture never seemed regular, instead, containing lots of laminates and dimples with the increase of the RE nanoparticles. The results also indicated that the optimal addition amount of the RE nanoparticles was 0.4%, under this composition, the microstructure and mechanical property achieved the best cooperation. In addition, through the study of erosion wear rate, when adding 0.4% RE nanoparticles into the HCCIs, the erosion wear rate got the minimum 0.32×10-3 g/mm2, which could increase 51.5% compared with that without any RE nanoparticles.