球化组织对AISI 420型钢淬回火特性及耐蚀性能的影响

对比研究了两种AISI 420型钢球化组织的平均粒径和圆整度，并对两种钢材进行了不同淬火和回火处理工艺.然后通过硬度测试、扫描电子显微镜（SEM）和X射线衍射仪（XRD）来比较球化组织对淬回火特性的影响，同时借助动电位极化曲线测试和质量分数3.5% NaCl溶液浸泡腐蚀来分析耐蚀性能的差异.结果表明:细小弥散的球化组织在淬火时可以提高AISI 420型钢的C元素的固溶量，提高了其淬硬性，但是会提高残留奥氏体的含量；尺寸更小的退火态碳化物可以使AISI 420型钢的基体在奥氏体化过程中溶解更多的Cr元素，从而使得其在淬回火后基体Cr含量更高，减小贫Cr区产生几率，最终显示出更好的点蚀抗力；更少的大尺寸的未溶碳化物在腐蚀环境中降低了点蚀形核几率，提高了AISI 420型钢的耐蚀性能.所以在250℃回火时，AISI 420型钢耐蚀性好且硬度高，在480℃回火后，耐蚀性最差.      

Influence of Cryoprocessing on Mechanism of Carbide Development in Cobalt-Bearing High-Speed Steel (M35)

Cryogenic treatment has been used commonly to high-speed tool steels to enhance the wear resistance of the materials. In the current research study, specimens of complex alloyed high-speed tool steel (M35) were hardened at 1473 K (1200 °C), triple tempered at 673 K (400 °C) and then cryogenically treated at 88 K (?185 °C) for varying lengths of period starting from 16 to 48 hours of cryosoaking followed by soft tempering at 373 K (100 °C). These treated specimens were studied as a function of cryosoaking period for their electrical resistivity, residual compressive stress, and its correlation with carbide density was established. TEM analysis indicated carbide size 0.156 to 1 μm, which confirms that the cryogenic treatment enhances the precipitation of finer carbides. Lower residual stresses in the higher carbide density regimes identified in 2D contour map were explained by the stress relaxation in the matrix through precipitation of incoherent carbides.     

深冷处理对NM500耐磨钢性能与磨损行为的影响

以一种自行设计的NM500级别耐磨钢为研究对象,利用冲击磨损试验,分析了深冷处理对其组织性能和磨损行为的影响。结果表明,NM500耐磨钢经深冷处理后,抗拉强度、硬度和冲击韧性均有提高,在淬火+深冷+回火处理后,最佳的综合力学性能可达抗拉强度1 910 MPa、硬度523HB、冲击韧性24.3 J/cm2,此时试验钢组织主要为马氏体,有Nb和Ti的碳化物析出。深冷处理通过残余奥氏体向马氏体转变,减少了不稳定相的含量,提升了试验钢的力学性能,从而使淬火+深冷+回火处理后的试验钢具有更高的耐磨性,此时的磨损机制以磨粒磨损为主,磨损形貌主要为犁沟、犁皱。而未经深冷处理的淬火+回火处理试验钢磨损机制以黏着磨损为主,磨损形貌主要为剥落坑和切削。     

Effect of Deep Cryogenic Treatment on Wear Resistance of AISI 52100 Bearing Steel

In this study, the effects of deep cryogenic treatment (DCT) on the wear resistance of AISI 52100 bearing steel were investigated. For this purpose, a number of bearing steel samples were held for different times (12, 24, 36, 48, 60 h) at deep cryogenic temperatures (?145 °C). The wear experiments were carried out in a ball–disk arrangement, by applying loads of 10 and 20 N and a sliding velocity of 0.15 m/s. After conducting the experimental studies, 36 h was found to be the optimal holding time. At this holding time, the wear rate and friction coefficient were decreased, while the hardness reached to maximum values. It was observed that DCT led to significant microstructural changes, which resulted in improved tribological properties.     

合金碳化物对剪板机刃具钢HM的组织和性能的影响

试验研究了HM钢(%:0.48C、5.1Cr、2.0Mo、1.0V、2.0W)和H13钢(%:0.38C、5.1Cr、1.4Mo、1.0V)的组织、冲击韧性和耐磨性。结果表明,HM钢中W、Mo元素含量较H13钢高,存在更多未溶碳化物阻碍奥氏体晶粒长大,细化晶粒,使平均冲击值比H13钢提高35.5%;HM钢高温回火时,马氏体基体弥散析出更多的碳化物,二次硬化效果显著,平均耐磨性比H13钢提高34.8%。     

Effect of molybdenum on the fracture characteristic of high-speed steel quenched matrix

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 K_Ic 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.     

Correlation between Microstructure and Mechanical Properties of High C‐Cr Cast Steel

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.     

Optimizing structure,toughness and wear performance of 15 % Cr cold work cast tool steel

A series of six Cr-, Cr + Mo-, Cr + Mo + V cold work cast tool steels were produced and investigated for microstructure, impact toughness and both experimental and industrial abrasive wear. Grain refinement of the steel matrix even in as-cast condition was obtained on using 2.3 % Mo + 0.9 % V and that ensured increasing impact toughness and abrasion resistance. An optimum impact toughness of about 85 J-cm^?2 was obtained in air quenched (970°C) and tempered (450°C) Mo + V containing steels in which area fraction of carbides reached 38 %. The abrasion resistance improved in case of steels tempered at 250°C and had fine grain structure.     

Comparison of microstructure and property of high chromium bearing steel with and without nitrogen addition

Microstructure and property of bearing steel with and without nitrogen addition were investigated by microstructural observation and hardness measurement after different heat treatment processing. Based on the microstructural observation of both 9Cr18 steel and X90N steel,it was found that nitrogen addition could effectively reduce the amount and size of coarse carbides and also refine the original austenite grain size. Due to addition of nitrogen,more austenite phase was found in X90N steel than in 9Cr18 steel. The retained austenite of X90N steel after quenching at 1 050 °C could be reduced from about 60% to about 7% by cold treatment at-73 °C and subsequent tempering,and thus finally increased the hardness up to 60 HRC after low temperature tempering and to 63 HRC after high temperature tempering. Furthermore,both the wear and corrosion resistance of X90N steel were found much more superior than those of 9Cr18 steel,which was attributed to the addition of nitrogen. It was proposed at last that nitrogen alloying into the high chromium bearing steel was a promising way not only to refine the size of both carbides and austenite,but also to achieve high hardness,high wear property and improved corrosion resistance of the stainless bearing steel.     

Effect of RE－Al－N on Structures and Properties of M2 Cast High Speed Steel

M 2highspeedsteel (M 2steel)isamaterialwidelyusedintoolsanddies[1,2 ] .M 2steelischarac terizedbyalongsolidificationrangeandcomplexeu tecticreactions ,whichresultsinsegregationofalloy ingelementsandformationofseveraldifferenttypesofcarbidesduringsolidification[3,4 ] .Forconventionallyprocessedhigh speedsteels ,itisinevitablethatacoarsecarbidenetworkwillbeformedduringsolidifi cation .Coarseprimarycarbidestendtoresultinun evencarbidebandsdistributionafterasubstantialamountofhotprocessing[5] .Ma…     

碳含量对高钒高速钢冲击磨损性能的影响

在含钒量约为10%的条件下,利用自行研制的WM-1型冲击磨损试验机研究碳含量在1.76%～3.65%范围内变化对高钒高速钢冲击磨损性能的影响.结果表明:含碳量低于2.3%,高钒高速钢的基体为铁素体,磨损机理主要是冲刷和塑性变形,耐磨性很差.含碳量达到2.3%后,基体以马氏体为主,耐磨性比低碳的高钒高速钢大幅提高.当碳含量达到3.0%时,耐磨性达到峰值.继续提高碳含量,晶界处以铬钼为主碳化物的量增加,冲击过程中裂纹在该处萌生,产生剥落现象,造成耐磨性下降.     

Tribological Performance of Austempered and Tempered Ductile Iron

Austempered ductile iron with its unique ausferritic structure is produced by an isothermal heat treatment process. Austempered ductile iron is a potential material to substitute for traditional steel castings and forgings in current industry due to its excellent mechanical properties. The tempering process is frequently used to enhance the ductility and toughness of a material and reduce residual stress. In this research, the phase transformation of austempered ductile iron was studied by applying various tempering temperatures with constant holding duration. It was found that the ausferritic structure was decomposed into dispersive cementite particles after receiving a tempering temperature of 538 °C or higher. The specific amount of retained austenite was analyzed by X-ray diffraction. The wear resistance of tempered austempered ductile iron was investigated by using a ball-on-disk sliding test configuration. The results were compared with conventional quenched and tempered ductile iron under equivalent hardness. Both austempered ductile iron and tempered austempered ductile iron samples had better wear resistance than quenched and tempered ductile iron. The results presented in this research can be utilized as a reference in the tempering treatment of austempered ductile iron material for future applications.     

Influence of Heat Treatment Parameters on the Carbide Morphology of PM High-Speed Steel HS 6-5-3-8

The influence of heat treatment parameters on the carbide morphology of the powder metallurgic high-speed steel HS 6-5-3-8 is examined. To that end, diverse heat treatment parameters are selected and applied by quenching dilatometry. In particular, different austenitizing temperatures, as well as an isothermal holding stage during quenching in the temperature regime of the transformation gap at temperatures between 450 and 600 °C, are produced. Extensive computer-aided image analysis is performed to investigate the carbide morphology. It is found that the circularity of the tungsten-rich M₆C carbides increases significantly after short holding times at a temperature of 550 °C due to the carbide precipitation from metastable and supersaturated austenite onto pre-existing carbides. Longer holding times lead to further growth of the carbides, while the circularity of the carbides does not change. It is further shown that the hardness of the isothermally treated material is increased, all other parameters being equal. Increased carbide circularity might be helpful for increasing the toughness while reaching the same hardness and wear resistance as the conventional heat-treated material. Moreover, it might be possible to enhance the austenitizing temperature with regain of positive carbide morphology properties during the isothermal holding stage. Thus, improved material properties could be achieved.     

The effect of the austenitizing heat-treatment variables on the fracture toughness of high-speed steel

The influence of austenitizing treatment and tempering on the fracture behavior of high-speed steel (DIN 1.3333) has been investigated. The fracture behavior has been characterized by determining the K _IC and J _IC values via the performance of modified compact tension (CT) and single edge notched (SEN) tests. The micromechanisms of crack initiation and propagation have been studied by metallographic examination of the fractured specimens. The results indicate that austenitizing conditions of temperature range 1050 °C to 1190 °C and time 0.25 to 6 minutes and tempering at 550 °C to 650 °C up to 150 minutes alter the microstructure and, subsequently, the fracture toughness. It was found that cracking occurs by nucleation at the interface of matrix/vanadium-enriched large carbides, where sulfur is segregated and where linkage of the microcracks bridges ductile ligament of voids at small Mo + W enriched carbides. The improvements of the fracture toughness and hardness by short austenitizing time of 15 to 75 seconds at 1190 °C are attributed to (1) the optimum distribution of a dense network of small carbides, (2) the lack of grain growth as the boundaries are pinned down by these small carbides, and (3) the retained austenite at a level up to 16 vol pct transformed to martensite.     

Wear and Fatigue Behaviour of Deep Cryogenically Treated H21 Tool Steel

The influence of deep cryotreatment before and after double tempering on hardness, wear rate, surface finish and fatigue limit of AISI H21 tool steel has been examined. In the present work, H21 tool steel has been subjected to heat treatment at 1195 °C, double tempering at 540 °C, deep cryotreatment at ??185 °C and soft tempering at 100 °C. The microstructure of samples has been characterized for the number density of carbides, hardness, wear rate, surface roughness and fatigue limit. The fatigue test has been carried out using a rotating bending fatigue machine to study the fatigue strength of the material. The obtained results show that the HTTC24 specimen reduces the wear rate by?≈?24% and surface roughness by?≈?21% with an increase in fatigue limit by?≈?13% compared to HTT specimens. This has been attributed to the increased number density of carbides and hardness; and approximately the complete conversion of retained austenite content into martensite.

     

Experimental investigation into microstructure,metallurgy and mechanical properties of AISI 422 stainless steel weldments produced using fibre laser welding direct forming

In this study, the Taguchi design method was used to determine the optimal parameters for the fibre laser welding direct forming (FLWDF) of AISI 422 stainless steel (SS) weldments for use in the repair of the tenons in steam turbine blades. Experiments were then conducted to investigate the microstructural characteristics and mechanical properties of the weldments. Our results show that the fusion zone of the AISI 422 SS weldments possesses a grain structure of significantly higher refinement than does the original AISI 422 SS. Moreover, tempering at 700°C for 2?h was found to enhance the hardness as well as the impact toughness of the weldments. Finally, mock-up trials of tenon repair were performed using the optimal FLWDF parameters. Our results reveal that the repaired tenons are able to withstand a tensile load of 30?kN in as-welded condition and up to 55?kN following tempering at 700°C for 2?h.     

Fracture toughness of aisi M2 high-speed steel and corresponding matrix tool steel

The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4 pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated. In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit 2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples is a maximum. The above described results were explained through changes in the microstructure and the fracture modes. CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI.     

The effects of surface hardening on fracture toughness of carburized steel

This research program was carried out to evaluate the effects of surface hardening on the fracture toughness of carburized steel. The materials AISI 8620 steel was machined into compact-tension (CT) specimens. The specimens were pack carburized at 930°C (1706°F) for different periods of time, cooled to ambient temperature and subsequently tempered at various temperatures for one hour. The fractured specimens were examined by hardness tests, metallography, X-ray diffraction analysis for retained austenite in the case, and scanning electron microscope fractographic analysis of the fracture surfaces. The experimental results revealed that theKIC values of the carburized, AISI 8620 steels were improved by the increase in case depth. Martensitic/tempered-martensitic structure in the case was the major constituent contributing to the improved toughness. The amount of retained austenite at the case increased as the thickness of the hardened layer increased. But retained austenite as well as large grain size were found to have adverse effects on fracture toughness of the carburized steel. The tempering temperature of 500°C (932°F) provided maximumKIC values. Higher tempering temperatures resulted in sharp decrease of fracture, toughness values. W{upeio}-Y{upoue} H{upo}, formerly a Graduate Student, in the Department of Materials Engineering Tatung Institute of Technology, is in compulsory 0 ROTC military service of Republic of China.     

Relationship Among Microstructure and Properties and Heat Treatment Process of Ultra-High Strength X120 Pipeline Steel

 The variation of heat treatments including directed quenching and tempering off-line after controlled rolling (DQT) and quenching off-line and tempering off-line after controlled rolling (RQT) with microstructure and mechanical properties of a low-carbon microalloyed steel was compared and analyzed. For DQT, the quenched steel was obviously banded microstructure, with increasing tempering temperature, lath martensite coarsened, the cusp carbide precipitated at grain boundaries, the yield strength fluctuated slightly, and the fracture-separation was obvious. The impact toughness was better in the steel tempered at 500 ℃ for 1 h. In RQT, with increasing tempering temperature, lath martensite degenerated, intragranular and intergranular finer precipitations with smaller than 30 nm precipitated and grew up and were distributed dispersedly, the stripe-like carbides were distributed at grain boundaries, and the yield strength and tensile strengthen decreased obviously. The impact toughness of RQT process was much better than that of DQT process, and the comprehensive mechanical properties were better for the steel tempered at 500 ℃ for 1 h of RQT process.     

Tempering-Induced Microstructural Changes in the Weld Heat-Affected Zone of 9 to 12 Pct Cr Steels and Their Influence on Sliding Wear

Increasing amount of tribological applications is working under alternating high/low temperature conditions where the material is subjected to temperature fatigue mechanisms such as creep, softening due to annealing, and at the same time must withstand mechanical wear due to sliding contact with pairing bodies. Steam turbine valves, gate valves, valve heads, stems, seats and bushings, and contacting surfaces of the carrier elements are some examples of such applications. The purpose of the present study is to evaluate the potential of X20 and P91 steels as materials for applications operating under combined effect of mechanical wear and alternating high/low temperature conditions. It was focused on how the microstructural changes occurring in the weld zone affect the wear properties of the selected materials. Generally, with longer tempering time and higher tempering temperature, the number of carbide precipitates decreased, while their relative spacing increased. Before tempering, the morphology of the steel matrix (grain size, microstructure homogeneity) governed the wear resistance of both steels, while after tempering wear response was determined by the combination of the number and the size of carbide particles. After tempering, in X20 steel larger number of stable M₂₃C₆ carbides was observed as compared with P91 steel, resulting in lower wear rates. It was observed that for both steels, a similar combination of number density and size distribution of carbide particles provided the highest wear resistance.