Fatigue behavior of a precipitation hardening Ni-Al-Cu medium carbon steel

The fatigue behavior of an Fe-0.3 wt pct C-4 wt pct Ni-1 wt pct Al-1 wt pct Cu precipitation hardening steel was investigated in three different heat treated conditions which give similar tensile strengths but different microstructures. One heat treatment produced a lightly tempered lath martensite having fine carbides and a high dislocation density. The other two heat treatments produced highly tempered martensite with coarse carbides, fine intermetallic precipitates and a relatively low dislocation density. The steel in the lightly tempered condition showed marked softening on strain cycling while the highly tempered conditions resulted in both hardening and softening. The lightly tempered structure had better low cycle fatigue resistance but the two highly tempered structures had better high cycle resistance. The dislocation substructure in the lightly tempered steel rearranges itself and accommodates plastic strain during cyclic deformation while the substructure in the highly tempered structures containing fine precipitates resists rearrangement. This difference is suggested as the reason for the differences in behavior. The three conditions show little variation in their resistance to fatigue crack propagation. However, the highly tempered, precipitate containing structures were much more resistant to fatigue crack initiation in notched specimens. Former Postdoctoral Research Associate, Department of Materials Science and Engineering, and Walter P. Murphy Professor of Materials Science and Engineering     

Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel

The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340 steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite. Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel.     

Load sharing between austenite and ferrite in a duplex stainless steel during cyclic loading

The load sharing between phases and the evolution of micro- and macrostresses during cyclic loading has been investigated in a 1.5-mm cold-rolled sheet of the duplex stainless steel SAF 2304. X-ray diffraction (XRD) stress analysis and transmission electron microscopy (TEM) show that even if the hardness and yield strength are higher in the austenitic phase, more plastic deformation will occur in this phase due to the residual microstresses present in the material. The origin of the microstresses is the difference in coefficients of thermal expansion between the two phases, which leads to tensile microstresses in the austenite and compressive microstresses in the ferrite. The microstresses were also found to increase from 50 to 140 MPa in the austenite during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. The cyclic loading response of the material was, thus, mainly controlled by the plastic properties of the austenitic phase. It was also found that initial compressive macrostresses on the surface increased from −40 to 50 MPa during the first 10³ cycles. After the initial increase of microstresses and macrostresses, no fading of residual stresses was found to occur for the following cycles. A good correlation was found between the internal stress state and the microstructure evolution. The change in texture during cyclic fatigue showed a sharpening of the deformation texture in the ferritic phase, while no significant changes were found in the austenitic phase.     

Fatigue behavior of a precipitation hardening Ni−Al−Cu medium carbon steel

The fatigue behavior of an Fe-0.3 wt pet C-4 wt pet Ni-1 wt pet Al-1 wt pet Cu precipitation hardening steel was investigated in three different heat treated conditions which give similar tensile strengths but different microstructures. One heat treatment produced a lightly tempered lath martensite having fine carbides and a high dislocation density. The other two heat treatments produced highly tempered martensite with coarse carbides, fine intermetallic precipitates and a relatively low dislocation density. The steel in the lightly tempered condition showed marked softening on strain cycling while the highly tempered conditions resulted in both hardening and softening. The lightly tempered structure had better low cycle fatigue resistance but the two highly tempered structures had better high cycle resistance. The dislocation substructure in the lightly tempered steel rearranges itself and accommodates plastic strain during cyclic deformation while the substructure in the highly tempered structures containing fine precipitates resists rearrangement. This difference is suggested as the reason for the differences in behavior. The three conditions show little variation in their resistance to fatigue crack propagation. However, the highly tempered, precipitate containing structures were much more resistant to fatigue crack initiation in notched specimens.     

组织形态对718塑料模具钢切削性能的影响

通过热处理制备出具有回火马氏体组织、下贝氏体组织以及粒状贝氏体组织的718钢,利用光学显微镜、扫描电子显微镜、X射线衍射仪、万能拉伸实验机比较其显微组织及力学性能。同时借助高速铣削实验及光学轮廓仪,研究力学性能以及组织结构对切削性能的影响。结果表明,当切削速度低于145 m·min?1时,贝氏体组织类型比回火马氏体组织更易切削,切削贝氏体组织比切削回火马氏体组织的刀具使用寿命高30%～40%。当切削速度高于165 m·min?1时,马氏体组织发生了加工软化现象,刀具使用寿命提高,切削性能上升。粒状贝氏体组织加工表面因为严重的刀具黏附而出现背脊纹路,马氏体组织具有最佳的切削表面粗糙度。综合考虑之下,三种组织的综合切削性能从高到低排序为:下贝氏体组织、马氏体组织、粒状贝氏体组织,采用300 ℃等温淬火工艺可以有效提升718塑料模具钢的综合切削性能。      

Strength and fatigue properties in medium carbon duplex steels

The effect of microstructure on strength and fatigue properties has been investigated in two medium carbon alloy steels (BS 817M40 and BS 835M30) by developing dual-phase, ferritic-martensitic microstructures. Hardness-strength relationships and fatigue resistance at comparatively high strength levels were investigated by producing various microstructures. Conventional quenching and tempering, intercritical annealing and step quenching were used to vary the proportion, morphology and distribution of the ferrite and martensite phases. The results of the present study show that both hardness and strength increase with increasing proportion of martensite and/or hardness of the second phase. The relationship between hardness or strength and martensite percent is not in good agreement with a simple “law of mixtures” but is compatible with a more rapid strength increase at high martensite contents. The dual phase microstructures from the present study show superior near threshold ΔK_TH values than normal tempered martensite. The results also show a high degree of correlation between Paris equation m values and fracture toughness K_IC, showing that for high m values K_IC is low and vice versa. The present experiments show that although crack initiation resistance in dual-phase steels is excellent crack propagation rates are higher than in quenched and tempered microstructures for a given ΔK.     

Niti and NiTi-TiC composites: Part II. compressive mechanical properties

The deformation behavior under uniaxial compression of NiTi containing 0, 10, and 20 vol pct TiC participates is investigated both below and above the matrix martensitic transformation temperature: (1) at room temperature, where the martensitic matrix deforms plastically by slip and/or twinning; and (2) at elevated temperature, where plastic deformation of the austenitic matrix takes place by slip and/or formation of stress-induced martensite. The effect of TiC particles on the stress-strain curves of the composites depends upon which of these deformation mechanisms is dominant. First, in the low-strain elastic region, the mismatch between the stiff, elastic particles and the elastic-plastic matrix is relaxed in the composites: (1) by twinning of the martensitic matrix, resulting in a macroscopic twinning yield stress and apparent elastic modulus lower than those predicted by the Eshelby elastic load-transfer theory; and (2) by dislocation slip of the austenitic matrix, thus increasing the transformation yield stress, as compared to a simple load-transfer prediction, because the austenite phase is stabilized by dislocations. Second, in the moderate-strain plastic region where nonslip deformation mechanisms are dominant, mismatch dislocations stabilize the matrix for all samples, thus (1) reducing the extent of twinning in the martensitic samples or (2) reducing the formation of stressinduced martensite in the austenitic samples. This leads to a strengthening of the composites, similar to the strain-hardening effect observed in metal matrix composites deforming solely by slip. Third, in the high-strain region controlled by dislocation slip, weakening of the NiTi composites results, because the matrix contains (1) untwinned martensite or (2) retained austenite, which exhibit lower slip yield stress than twinned or stress-induced martensite, respectively. K.L. FUKAMI-USHIRO, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology D. MARI, formerly Postdoctoral Fellow, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

The role of the constituent phases in determining the low temperature toughness of 5.5Ni cryogenic steel

Ferritic Fe-Ni steels that are intended for service at low temperature are usually given an intercritical temper as the final step in their heat treatment. The temper dramatically decreases the ductile-brittle transition temperature, T_B. Its metallurgical effect is to temper the lath martensite matrix and precipitate a distribution of fine austenite particles along the lath boundaries. Prior research suggests that the low value of T_B is a consequence of the small effective grain size of the ferrite-austenite composite. The present research was done to test this suggestion against the counter-hypothesis that the low T_B is due to the inherent toughness of the constituent phases. The approximate compositions of the tempered martensite and precipitated austenite phases in the composite microstructure of tempered 5.5Ni steel are known from STEM analysis. Bulk alloys were cast with these two compositions. Their mechanical properties were measured after heat treatment and compared to those of the parent alloy in the toughened ‘QLT’ condition. Both of the constituent phases are brittle at low temperature. It follows that the outstanding low-temperature toughness of the tempered alloy cannot be attributed to the inherent properties of the constituent phases, but must reflect their cooperative behavior in the composite microstructure. The austenitic bulk alloy was also used to investigate the stability of the precipitated austenite phase. The thermomechanical stability of the bulk alloy approximates that of the precipitated austenite within tempered 5.5Ni steel. This result is consistent with previous data, and supports the conclusion that the stability of the precipitated austenite is determined mainly by its chemical composition.     

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Effect of partitioning on microstructure and properties of a medium carbon Ti Mo bearing high strength Q&P steel

In order to investigate the effect of partitioning procedures on microstructure and properties of a medium carbon Ti Mo bearing steel, the salt bath experiment, field emission SEM, TEM and tensile tests as well as wear tests were utilized to clarify the microstructural evolution and property changes treated by different partitioning parameters. The results show that the microstructures consist of tempered lath martensite, cementite and (Ti, Mo)C particles. With prolonging partitioning time or increasing partitioning temperature, the amount of laths martensite decreases and thicker martensite plates with blunt boundaries appear. In addition, with increasing partitioning temperature from 310℃ to 400℃, the tensile strength, hardness and low temperature toughness are simultaneously decreased, and the reduction reaches up to about 300MPa, 100HV and 20J, respectively. Finally, the wear performance of the samples partitioned below Ms is obviously better than that of samples partitioned above Ms. The morphology of worn surface of samples partitioned below Ms is characterized by plastic fatigue morphology, while the worn surfaces of samples partitioned above Ms are mainly dominated by grooves.     

配分工艺对中碳Ti-Mo高强Q&P钢组织和性能的影响

摘要：采用盐浴实验、扫描电镜、透射电镜、拉伸实验和磨损实验等手段，研究了配分工艺对中碳Ti Mo钢组织和性能的影响，分析了不同配分工艺处理下的组织演变和性能变化。结果表明，显微组织主要由回火马氏体、渗碳体、(Ti，Mo)C粒子组成。随着配分时间的延长和配分温度的升高，板条马氏体数量减少，马氏体板条厚度增加，边界钝化。此外，随着配分温度从310℃提高至400℃，抗拉强度、硬度和低温冲击韧性同时下降，分别降低约250MPa、56HV和15J。最后，Ms以下温度配分（310℃）试样的耐磨损性能明显优于Ms以上温度配分（400℃）试样。Ms以下温度配分试样磨损表面形貌以塑性变形为主，Ms以上温度配分试样磨损表面以犁沟为主。     

共析轨钢塑性应变与应变疲劳关系的研究

用全反向恒应变幅试验方法,研究了C-Mn轨钢珠光体及回火索氏体的应变疲劳行为,探讨了塑性应变在应变疲劳中的作用。试验结果表明,轨钢的疲劳是塑性应变控制过程。大应变幅时,疲劳寿命的急剧缩短与塑性应变幅的快速增长有关,减小片间距会降低塑性应变幅在总应变幅中所占比例;抗拉强度相等时,因循环软化而致的塑性应变幅增大及较小的可允许应变范围是回火索氏体疲劳寿命低于珠光体的主要原因。     

The effect of phase continuity on the fatigue and crack closure behavior of a dual-phase steel

The effect of phase continuity on the low cycle fatigue and fatigue crack growth behavior of a Fe-C-Mn dual-phase steel has been investigated. Two microstructures, one consisting of continuous ferrite and the other continuous martensite, were examined. Although there was no difference in the low cycle fatigue lives between the two microstructures, the continuous martensite structure exhibited an extremely high fatigue threshold value of 20 MPa m^1/2, compared to 16 MPa m^1/2 for the continuous ferrite microstructure. A major effect of phase continuity has also been found in the crack closure levels during fatigue crack propagation studied over three decades of crack growth rates. The continuous martensite microstructure exhibited much higher closure levels due to the martensite constraining the plastic deformation in the ferrite and bearing a larger portion of the applied cyclic load. This effect is similar to the extrinsic toughening phenomenon cited in the literature. After accounting for the closure levels the intrinsic or effective fatigue crack growth rates are similar for the two microstructures. These intrinsic thresholds are predicted by employing experimentally obtained low cycle fatigue parameters and the ferrite grain size.     

Microstructural Evolution of AerMet100 Steel Coating on 300M Steel Fabricated by Laser Cladding Technique

In this paper, the process of coating AerMet100 steel on forged 300M steel with laser cladding was investigated, with a thorough analysis of the chemical composition, microstructure, and hardness of the substrate and the cladding layer as well as the transition zone. Results show that the composition and microhardness of the cladding layer are macroscopically homogenous with the uniformly distributed bainite and a small amount of retained austenite in martensite matrix. The transition zone, which spans approximately 100 μm, yields a gradual change of composition from the cladding layer to 300M steel matrix. The heat-affected zone (HAZ) can be divided into three zones: the sufficiently quenched zone (SQZ), the insufficiently quenched zone (IQZ), and the high tempered zone (HTZ). The SQZ consists of martensitic matrix and bainite, as for the IQZ and the HTZ the microstructures are martensite + tempered martensite and tempered martensite + ferrite, respectively. These complicated microstructures in the HAZ are caused by different peak heating temperatures and heterogeneous microstructures of the as-received 300M steel.     

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.     

连铸与模铸高铁车轴钢的高周疲劳破坏行为

连铸车轴钢能否达到模铸车轴钢的性能水平是其能否应用的一个关键。对此,采用旋转弯曲疲劳试验及疲劳裂纹扩展速率试验对比研究了连铸与模铸工艺生产的高铁车轴钢的高周疲劳破坏行为。结果表明,工业试制的连铸车轴钢的强度和疲劳极限均低于模铸车轴钢,且前者的疲劳裂纹扩展速率略高于后者。疲劳断口分析表明,疲劳断裂大部分起源于试样表面基体。微观组织分析表明,尽管两者的微观组织均为高温回火马氏体,但连铸车轴钢中原奥氏体晶粒尺寸及碳化物均略大于模铸车轴钢。金相评级法及夹杂物极值统计法的结果均表明,连铸车轴钢中的夹杂物尺寸明显大于模铸车轴钢。因此,为了以连铸工艺取代模铸工艺,还需要进一步优化连铸车轴钢的成分、冶金生产和热处理等工艺,以获得优良的冶金质量和组织性能。     

Structure and mechanical properties of tempered martensite and lower bainite in Fe−Ni−Mn−C steels

The structure and mechanical properties of tempered martensite and lower bainite were investigated in a series of high purity 0.25 pct C steels with varying amounts of nickel and manganese. The martensites in 0.25 C-5 Ni?Fe and 0.25 C-3 Mn?Fe alloys were mainly untwinned, while those in 0.25 C-5 Ni-7 Mn?Fe and 0.25 C-7 Mn?Fe alloys were heavily twinned. Manganese appears to promote carbide precipitation along the lath boundaries in tempered martensite. At equivalent yield and ultimate tensile strength levels, the tempered martensite of lower manganese steels showed better impact toughness than the tempered martensite of higher manganese steels. The impact toughness (compared at similar strength levels) of untwinned tempered martensite of 0.25 pct C steel with Widmanstatten precipitation of carbide was higher than that of lower bainite, which showed unidirectional carbides. The reasons for the difference in impact toughness between the alloys, and also between the structures are rationalized in terms of internal twinning, grain boundary precipitation and carbide morphology together with other microstructural features.     

The influence of martensite shape,concentration, and phase transformation strain on the deformation behavior of stable dual-phase steels

A continuum model is developed to examine the influence of martensite shape, volume fraction, phase transformation strain, and thermal mismatch on the initial plastic state of the ferrite matrix following phase transformation and on the subsequent stress-strain behavior of the dual-phase steels upon loading. The theory is developed based on a relaxed constraint in the ductile matrix and an energy criterion to define its effective stress. In addition, it also assumes the martensite islands to possess a spheroidal shape and to be randomly oriented and homogenously dispersed in the ferrite matrix. It is found that for a typical water-quenched process from an intercritical temperature of 760 °C, the critical martensite volume fraction needed to induce plastic deformation in the ferrite matrix is very low, typically below 1 pct, regardless of the martensite shape. Thus, when the two-phase system is subjected to an external load, plastic deformation commences immediately, resulting in the widely observed “continuous yielding” behavior in dual-phase steels. The subsequent deformation of the dual-phase system is shown to be rather sensitive to the martensite shape, with the disc-shaped morphology giving rise to a superior overall response (over the spherical type). The stress-strain relations are also dependent upon the magnitude of the prior phase transformation strain. The strength coefficienth and the work-hardening exponentn of the smooth, parabolic-type stress-strain curves of the dual-phase system also increase with increasing martensite content for each selected inclusion shape. Comparison with an exact solution and with one set of experimental data indicates that the theory is generally within a reasonable range of accuracy. Formerly Visiting Professor, Department of Mechanical and Aerospace Engineering, Rutgers University     

Short‐term Creep Behavior of an X 37 Cr Mo V 5‐1 Hot‐work Tool Steel with almost Bainitic and fully Martensitic Microstructures

In this study two different heat treatments were conducted on an X 37 Cr Mo V 5‐1 hot‐work tool steel, resulting either in a tempered fully martensitic matrix or a matrix almost consisting of tempered bainite. Short‐term creep tests were performed at a high stress level of 800 MPa and at temperatures in the range from 450 °C to 500 °C. Creep specimens consisting of a tempered fully martensitic microstructure exhibited a three times longer creep‐to‐rupture time, than those consisting of a tempered almost bainitic microstructure. Microstructural investigations of creep specimens were performed by transmission electron microscopy. Results of these investigations revealed that due to a lower cooling rate, which is necessary to form bainite, the tempered bainitic microstructure consists of large former bainitic plates, whereas tempered martensite shows fine former martensitic laths. Tempered bainite also exhibits a higher number density of large M₃C, M₇C₃ and MC carbides than tempered martensite. Small M₂C carbides appear in both microstructures in the same quantity, however, nanometer‐sized MC carbides could only be found in tempered martensite. Thus poor short‐term creep behavior of the tempered almost bainitic microstructure can be explained by the lesser amount of strengthening relevant precipitates, a smaller size‐effect due to distance of bainitic interfaces as well as lower solid solution hardening.     

Effect of Heat Treatment on Microstructure and Property of Cr13 Super Martensitic Stainless Steel

 The microstructures and mechanical properties of Cr13 super martensitic stainless steel after different heat treatments were studied. The results show that the structures of the steel after quenching are of lath martensite mixed with a small amount of retained austenite. With the raising quenching temperature, the original austenite grain size increases and the lath martensite gradually becomes thicker. The structures of the tempered steel are mixtures of tempered martensite and reversed austenite dispersed in the martensite matrix. The amount of reversed austenite is from 754% to 2249%. After different heat treatments, the tensile strength, the elongation and the HRC hardness of the steel are in the range of 813-1070 MPa, 101%-212% and 2133-3237, respectively. The steel displays the best comprehensive mechanical properties after the sample is quenched at 1050 ℃ followed by tempering at 650 ℃.