Quench embrittlement of hardened 5160 steel as a function of austenitizing temperature

Charpy V-notch (CVN) specimens from experimental heats of 5160 steel containing 0.001 and 0.034 mass pct phosphorus were austenitized at temperatures between 830 °C and 1100 °C, quenched to martensite, and tempered at temperatures between 100 °C and 500 °C. Scanning electron microscopy (SEM) was used to characterize the fracture surfaces of tested CVN specimens and carbide formation on prior austenite grain boundaries. Quench embrittlement, the susceptibility to intergranular fracture in as-quenched and low-temperature tempered high-carbon steels due to cementite formation as affected by phosphorus segregation on austenite grain boundaries, developed readily in specimens of the high phosphorus steel austenitized at all temperatures. The low phosphorus steel developed quench embrittlement only after austenitizing at 1100 °C. Intergranular fractures correlated with low room-temperature CVN impact toughness. The results are discussed with respect to the dissolution of carbides during austenitizing and the effect of phosphorus on grain boundary, carbide formation, and stability.     

Charpy-impact-toughness prediction using an “effective” grain size for thermomechanically controlled rolled microalloyed steels

Thermomechanically controlled rolling of steel plate can involve substantial straining in the intercritical temperature region, which may result in the final ferrite grains not fully recrystallizing, and, hence, the presence of low-angle grain boundaries. It is shown in this article that a Nb-microalloyed thermomechanically controlled rolled (TMCR) steel can contain a high proportion of low-angle grain boundaries (the extent depending on the thermomechanically controlled rolling schedule) and that during toughness testing, the crack front ignores boundaries with less than a 12 deg misorientation. Thus, the average microstructural unit experienced by the crack front (i.e., the cleavage facet) is significantly larger than the average metallographic, two-dimensional grain size. Consequently, use of the metallographic grain size gives a poor prediction of the impact transition temperature (ITT) and fracture stress for these steels. It is also shown that the micromechanism of crack initiation and propagation involves grain-boundary carbides and groups of closely aligned grains that act as single “effective” grains.     

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.     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

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.     

Effect of long-term service exposure at elevated temperature on microstructural changes of 5Cr-0.5Mo steels

Effects of long-term service exposure at elevated temperature on microstructural changes have been studied for both virgin and service-exposed process heater tube pipes of 5Cr-0.5Mo steels used in oil refineries. Samples selected for this study had experienced a nominal temperature range of 450 °C to 500 °C for about 20 to 25 years. Two different initial virgin microstructures were taken and designated by steel A and steel B. The virgin microstructure of steel A exhibited fine platelets of fibrous or hairlike M₂C carbides within the ferrite grains and occasionally irregularly shaped M₂₃C₆, both along the grain boundaries and at the grain interiors, and very few spheroidally shaped M₃C, either along the grain boundaries or at the grain interiors. The size, shape, position, distribution, and type of carbides in virgin steel A changed significantly due to 220,000 hours of service exposure in the temperature range of 450 °C to 500 °C. Massive M₂₃C₆ carbides precipitated along the grain boundaries. In addition, regular geometrically shaped M₂₃C₆ carbides, such as hexagonal, square, and triangular type, were observed to form at the grain interiors. The virgin steel B microstructure exhibited predominantly M₂₃C₆ carbides, either along the grain boundaries or at the lath boundaries. Occasionally, fine platelets of M₂C carbides were also observed within the laths. The position, shape, distribution, and type of carbides did not change significantly due to 172,000 hours of service exposure in the temperature range of 450 °C to 500 °C. The average interparticle spacings of the carbides increased from 0.35 to 1.2 μm due to 172,000 hours of exposure.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Nb对2000 MPa桥索钢再加热过程中组织变化的影响

针对不同Nb含量的2种桥索钢,采用热膨胀仪、光学显微镜、扫描电子显微镜和硬度测试仪对其在箱式电阻炉连续加热过程中的组织演变和水冷淬火后的硬度进行了对比分析。结果表明:Nb元素可以细化桥索钢的原始组织,使其存在大量的铁素体和渗碳体的晶界,在连续加热过程中的开始阶段提供更多的奥氏体形核位置,使得奥氏体逆共析转变的起始温度降低,而终了温度升高,逆共析转变区域增大。同时,Nb元素形成的碳化物在加热阶段对奥氏体晶粒的长大具有拖拽作用,降低桥索钢在奥氏体形核后的长大速度,使得淬火后得到马氏体的硬度值降低,因此需要较高的温度来溶解合金碳化物使桥索钢充分奥氏体化。     

Role of Tungsten in the Tempered Martensite Embrittlement of a Modified 9 Pct Cr Steel

The effect of tempering on the mechanical properties and fracture behavior of two 3 pct Co-modified 9 pct Cr steels with 2 and 3 wt pct W was examined. Both steels were ductile in tension tests and tough under impact tests in high-temperature tempered conditions. At T ≤ 923 K (650 °C), the addition of 1 wt pct W led to low toughness and pronounced embrittlement. The 9Cr2W steel was tough after low-temperature tempering up to 723 K (450 °C). At 798 K (525 °C), the decomposition of retained austenite induced the formation of discontinuous and continuous films of M₂₃C₆ carbides along boundaries in the 9Cr2W and the 9Cr3W steels, respectively, which led to tempered martensite embrittlement (TME). In the 9Cr2W steel, the discontinuous boundary films played a role of crack initiation sites, and the absorption energy was 24 J cm^?2. In the 9Cr3W steel, continuous films provided a fracture path along the boundaries of prior austenite grains (PAG) and interlath boundaries in addition that caused the drop of impact energy to 6 J cm^?2. Tempering at 1023 K (750 °C) completely eliminated TME by spheroidization and the growth of M₂₃C₆ carbides, and both steels exhibited high values of adsorbed energy of ≥230 J cm^?2. The addition of 1 wt pct W extended the temperature domain of TME up to 923 K (650 °C) through the formation of W segregations at boundaries that hindered the spheroidization of M₂₃C₆ carbides.     

Long-time isothermal temper embrittlement in Ni-Cr-Mo-V steels

Four commercial purity Ni-Cr-Mo-V steels of closely comparable bulk chemistry and grain size, but tempered to various strength levels, were embrittled by exposure at 600°, 750°, and 850°F for times up to 35,000 hr. Maximum temper embrittlement occurred at 850°F in all steels. Severe cases of embrittlement resulted in a marked decrease in tensile ductility and an intergranular tensile fracture. Auger electron emission analysis showed that P, Sn, Ni, and Cr were segregated at prior austenite boundaries in the steels exposed to 750° and 850°F. Increased segregation of phorphorus and tin was always accompanied by increased segregation of nickel and chromium. The severity of grain boundary segregation increased with increasing values of fracture transition temperature. Despite comparable bulk chemistry and grain size, the degree of segregation was different in different steels. Under exposure conditions causing severe embrittlement, the FATT values displayed a strong dependence on the strength level of the steel. In a giyen steel, while the composition and morphology of carbides at austenite boundaries were the same as in the matrix, the density and size of carbides were much higher at the austenite boundaries. The preference of these boundaries as fracture sites would seem to arise from two considerations, namely, a high degree of impurity and alloy element segregation and the fact that the density and size of carbides at these boundaries is higher than that in the matrix. On educational leave from Westinghouse Research Laboratories, Pittsburgh, Pa.     

10MnCuPTi耐大气腐蚀钢的韧性研究

P是耐大气腐蚀钢中的重要元素之一 ,但是 P在钢中极易偏聚在晶界 ,导致钢的韧性下降 .本文利用俄歇电子能谱分析研究了热轧状态下 P的行为 ,并通过金相和扫描电镜观察分析了1 0 Mn Cu PTi钢在热轧和正火后的显微组织 .实验结果表明 ,钢中 P有少量偏聚 ,但并未对钢的韧性产生影响 ,而较大的铁素体晶粒及沿晶分布的连续网状游离渗碳体是导致韧性下降的直接原因 ;采取适当的热轧工艺及轧后正火处理可以消除晶界渗碳体 ,改善韧性 .     

Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated heat-affected zones of Mn-Mo-Ni low-alloy steels

This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M₂C carbides was greatly increased and the number of coarse M₃C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M₂C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.     

Structural and phase states in high-quality rail

The structural and phase states and dislocational substructure in high-quality bulk-quenched rail are assessed quantitatively by transmission electron diffraction microscopy. On the basis of the morphological features, the following structural components of the rail steel are identified: plate pearlite (68%); mixed ferrite–carbide grains (28%); and structure-free ferrite grains (4%). Analysis of the flexural extinction contours shows that the stress concentrators in the steel are the boundaries between cementite plates within the pearlite grains; the boundaries between the pearlite grains and the ferrite grains; and the boundaries between globular particles of secondary phase and the ferrite matrix. The particle–matrix boundaries are the most significant stress concentrators and may be regarded as the primary sites of crack formation.     

Effect of non-metallic inclusions on fracture toughness of tool steels

The aim of these investigations was first of all to evaluate the fracture toughness (K_lc) changes of the hot-work tool steels depending on the non-metallic inclusions (NMI) volume fraction (melting technology). The tests were carried out on two types of the hot-work tool steels, i. e. H13 and H11 according to AISI. As a result of these investigations, supplemented by the detailed fractographic analysis, it has been revealed that uniform arrangement of NMI in the structure can be considered as harmless for the fracture toughness of tool steels. At high steel hardness values, the NMI, because of their action with a very small plastic strain zone, can be treated as natural obstacles in the crack propagation. At low hardness values of tool steels, achieved as a result of tempering at high temperatures, the role of NMI in the process of crack formation of these steels is limited by carbides precipitated from martensite. The micro-voids are formed round these carbides, which, connecting earlier than the voids formed round NMI, set the path of cracking and determine the steel fracture toughness.     

Recovery,recrystallization and grain growth in 0.03 C, 1.8 Si, 0.3 Al steels with and without addition of 0.05% of antimony

The investigation was carried out on two laboratory steels elaborated from identical base materials, one with 0.05% Sb and an industrial steel as-delivered and after decarburisation. Cold rolled 0.5 mm sheets were prepared by laboratory rolling and investigated after annealing in temperature range 550 to 800°C for 0.5 to 60 m. Antimony has no effect on recovery in temperature range 550 to 625°C. Rare recrystallization nuclei were found at grain boundaries only in the decarburized steel, in all other steels nucleii appeared and grew only inside of deformed grains. At recrystallization finished grains were coarser in antimony and decarburized steels. The explanation is that less numerous nuclei grew for a longer time in the deformed matrix. The size of recrystallized grains was proportional to the square root of the annealing time. Recrystallized grain growth was faster in antimony-free laboratory and in decarburized industrial steels, while the growth activation energy is very similar in antimony and decarburized steels. The growth activation energy was greater in antimony-free laboratory and in as-delivered industrial steels. It was similar to the activation energy of part of the recovery and near to the activation energy of iron self-diffusion in ferrite. No difference was found in grain growth topology between the antimony and the comparative laboratory steels. Results indicate a similar effect of the presence of 0.05% of antimony and the decreased carbon content in steel.     

Tempered martensite embrittlement and intergranular fracture in an ultra-high strength sulfur doped steel

This paper reports a study of tempered martensite embrittlement in a Ni-Cr steel doped with 0.01 wt pct S. The segregation of sulfur to the grain boundaries and the associated embrittlement of this material are very dependent upon the austenitizing temperature. If the austenitizing temperature is below 1050 °C very little embrittlement and very little intergranular fracture are observed because sulfur remains precipitated as chromium sulfide. At higher austenitizing temperatures the sulfides dissolve and sulfur segregates to the grain boundaries. Because of the high bulk content, the sulfur concentration at the grain boundaries becomes great enough for the sulfides to reprecipitate there. This leads to low energy intergranular ductile fracture. However, some sulfur remains unprecipitated at the boundary and can lower the cohesive strength across the boundary. When plate-like cementite precipitates at the grain boundary during tempering heat treatments at 300 to 400 °C, the combination of the carbides and the unprecipitated sulfur causes intergranular fracture and tempered martensite embrittlement.     

Microstructure and fracture of 52100 steel

The fracture behavior of 52100 steel hardened and tempered to R_C62 has been investigated as a function of austenitizing over the temperature range from 800 to 1100°C. Specimens were homogenized at 1150°C and either furnace cooled or isothermally transformed at 580°C to produce a pearlitic microstructure prior to austenitizing for hardening. Furnace-cooled specimens developed a proeutectoid carbide network that did not dissolve during subsequent austenitizing below A_cm . The residual proeutectoid carbides and the carbide-free martensite-austenite structure between them controlled fracture and produced K_IC of 19 MPa \ m^1/2, the highest determined in this investigation. The specimens isothermally transformed prior to austenitizing below A_cm produced a microstructure of fine spherical carbides dispersed throughout a fine martensitic matrix and did not contain residual proeutectoid carbides. The transgranular fracture of the latter specimens by microvoid coalescence around the closely spaced spherical carbides resulted in the lowest values of fracture toughness, 14 to 16 MPa\ m^1/2, determined in these experiments. Austenitizing above A_cm caused solution of all carbides, a gradual coarsening of the austenitic grain size, a transition to plate martensite, and an increase in retained austenite. Fracture toughness increased slightly with increasing austenitizing temperature above A_cm despite the fact that fracture propagated primarily along the austenitic grain boundaries. The improved fracture toughness, verified by scanning electron microscopy of the fatigue crack-overload fracture interface, is believed to be caused in part by transgranular crack propagation during the first stages of crack extension that are most important in determining K_1C.     

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.     

The isothermal austenite-ferrite transformation in some deformed vanadium steels

The effect of vanadium on the isothermal austenite-ferrite transformation, between 725 °C and 775 °C, of a hot-deformed microalloyed steel has been studied by examination of the microstructure and measurement of the volume fraction of ferrite in specimens quenched from the reaction temperature. The accompanying precipitation was studied by transmission electron microscopy of thin foils and carbon extraction replicas and by electron energy-loss spectroscopy. Very early in the transformation a continuous band of fine-grained ferrite forms at austenite grain boundaries. After some time some of these grains coarsen to form large equiaxed ferrite grains. It is found that vanadium has no effect on the time to the start of coarsening but thereafter accelerates the rate of formation of ferrite. Interphase precipitation of VN occurs throughout the transformation in the vanadium steels and this is thought to influence the rate at which the ferrite coarsens at the lower temperatures (750 ° and 725 °C) in the range studied.     

Influence of Tempering Time on the Microstructure and Mechanical Properties of AISI M42 High-Speed Steel

AISI M42 high-speed steel is prone to fracture as a result of its brittle martensitic microstructure together with abundant carbides located at the grain boundaries. In this study, a series of property tests including hardness, impact toughness, and wear loss were performed to study the effect of tempering conditions on the mechanical properties of AISI M42 high-speed steel over holding time ranging from 1 to 20 hours. The effects of the tempering time on the characteristics and growth of carbides were also investigated. The results indicated that carbides in the experimental steels were obviously coarsened when the tempering time exceeded 4 hours. The dimension of the carbides increased, while the volume fraction decreased with the increasing tempering time, and the grain sizes were significantly augmented due to the reducing of small carbides. Moreover, the dislocation density decreased with the increasing tempering time, which led to the reducing of the yield stress of high-speed steel. An appropriate holding time (4 hours) resulted in fine-scale secondary carbides and a smaller grain size, which efficiently improved the impact toughness and wear resistance simultaneously. Nevertheless, a prolonged tempering time (>?4 hours) promoted the coarsening and coalescence of carbides, which were detrimental to the impact toughness and wear resistance. Consequently, the formation of fine-scale secondary carbides is the major influential factor to improve both the wear resistance and impact toughness.