The influence of Mo on P-lnduced temper embrittlement in Ni-Cr steel

The effects of the addition of Mo on the temper embrittlement susceptibility of a P-doped Ni-Cr steel were studied. It is shown that P-induced temper embrittlement can be eliminated for aging times up to 1000 h at 475 and 500°C, presumably by the scavenging effect of Mo on P, if the Mo : P ratio is high enough and if the Mo is not precipitated by C. The latter reaction, which allows temper embrittlement to proceed, can occur more rapidly as the aging temperature or C content is raised. Formerly Research Fellow in the Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with Research Staff, LRSM, University of Pennsylvania     

The effect of an intercritical heat treatment on temper embrittlement of a Ni-Cr-Mo-V rotor steel

The effect of an intercritical heat treatment on tempor embrittlement has been investigated for a rotor steel containing 0.25 pct C, 3.5 pct Ni, 1.7 pct Cr, 0.5 pct Mo, 0.1 pct V, and deliberate additions of phosphorus, tin, or antimony. Both martensitic and bainitic steels were held at the intercritical temperature of 1380°F (750°C) for times up to 40 h and were then quenched or cooled to obtain martensitic or bainitic transformation. The steels were then tempered, followed by water quenching or step cooling from the tempering temperature. The residual ferrite maintained a fine plate-like shape even after 40 h at the intercritical temperature. Embrittlement induced by step cooling from the final tempering was mark edly reduced by the intercritical treatment as compared to the embrittlement observed after conventional heat treatment; for example, AFATT, the increase in the Charpy V-notch 50 pct shear fracture transition temperature caused by step cooling, was reduced by at least 80°F (45°C) as a result of the intercritical treatment of steels containing 0.02 pct P. Molybdenum effectively reduced AFATT in intercritlcally heat-treated steels as well as in conventionally treated steels. Possible mechanisms for reducing temper embrittlement with the intercritical treatment are discussed.     

Control of temper embrittlement in Ni-Cr-Mo-V steel by combining intercritical and low temperature austenitizing heat treatments

It is demonstrated in three heats of a Ni-Cr-Mo-V steel that dramatic reductions in both initial transition temperatures and susceptibility to temper embrittlement may be achieved by intercritical heat treatments. However, it appears that the temperature control necessary to ensure sufficient strength in large forgings of this class of steel will preclude the use of the treatment as a substitute for a complete austenitize. An approach to exploit the intercritical treatment as a conditioning procedure is described following the discovery that much of the reduced embrittlement susceptibility is remembered after a low temperature austenitize. Thus, by combining an intercritical and low temperature austenitize, the available temperature range for the intercritical treatment is widened because the minimum strength requirement is eliminated. Also, the full austenitize allows conventional tempering treatments to be used. The results are supported by fractography, metallography and Auger analysis.     

Effect of trace elements, molybdenum, and intercritical heat treatment on temper embrittlement of 2-1/4Cr-1 Mo steel

To minimize the temper-embrittlement susceptibility of 2 l/4Cr-lMo steel, minor changes in trace-element content and/or heat treatment will provide an improvement. Specimens containing two trace-element and six molybdenum levels were quenched and tempered to 100 ksi (690 N/mm²) tensile strength and embrittled by either isothermal aging at 900‡F (480°C) for 1000 h or step cooling. When 33 ppm of P, Sn, As, and Sb are present, fracture appearance transition temperature (FATT) remains at about -130°F (-90°C) between 0.5 and 1.4 pct Mo. When 405 ppm of the trace elements are present, embrittled FATT reaches a minimum -50‡F (-45°C). Thus, temper embrittlement can be suppressed by maintaining low trace-element concentrations. Intercritical heat treatment at 1575°F (855°C) also reduces temper embrittlement in air-cooled l/2 in. (13 mm) thick plates which simulate water-quenched heavy plates.     

Effect of trace elements,molybdenum, and intercritical heat treatment on temper embrittlement of 2-1/4Cr-1 Mo steel

To minimize the temper-embrittlement susceptibility of 2 l/4Cr-lMo steel, minor changes in trace-element content and/or heat treatment will provide an improvement. Specimens containing two trace-element and six molybdenum levels were quenched and tempered to 100 ksi (690 N/mm²) tensile strength and embrittled by either isothermal aging at 900?F (480°C) for 1000 h or step cooling. When 33 ppm of P, Sn, As, and Sb are present, fracture appearance transition temperature (FATT) remains at about -130°F (-90°C) between 0.5 and 1.4 pct Mo. When 405 ppm of the trace elements are present, embrittled FATT reaches a minimum -50?F (-45°C). Thus, temper embrittlement can be suppressed by maintaining low trace-element concentrations. Intercritical heat treatment at 1575°F (855°C) also reduces temper embrittlement in air-cooled l/2 in. (13 mm) thick plates which simulate water-quenched heavy plates.     

Temper embrittlement of Ni-Cr steel by Sn

Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia,PA Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with the Research Staff, LRSM, University of Pennsylvania     

The influence of temper embrittlement on the stress corrosion susceptibility of Fe-3 wt.% Ni alloys

The influence of a temper embrittling heat treatment on the intergranular stress corrosion susceptibility of an Fe-3 wt.% Ni alloy containing minor additions of tin or phosphorus has been investigated. Microanalysis of the grain boundary regions has been carried out using a scanning transmission electron microscope (STEM) fitted with an energy dispersive X-ray spectrometer. Measured changes in stress corrosion cracking behaviour, resulting from embrittlement at 823 K, have been correlated with changes in the electrochemical properties of the grain boundary regions. Crack growth has been interpreted in terms of an anodic dissolution process.     

Effects of austenitisation heat treatment on the fracture resistance and temper embrittlement of MnMoNi steels

The resistance to ductile and brittle fracture of four experimental melts of MnMoNi steel containing varying levels of sulphur, copper and phosphorus has been examined as a function of austenitisation heat treatment, with and without subsequent ageing at 500°C following tempering at 650°C. Fracture resistance was assessed by Charpy impact tests, fracture modes were studied using the scanning electron microscope, grain boundary segregation was quantified from Auger spectroscopy, and boron distribution determined by boron autoradiography. The results indicate that austenitisation heat treatment strongly influences the ductile-brittle transition temperature (DBTT) and upper shelf fracture energy (USE) in the quenched and tempered condition. The subsequent susceptibility to temper embrittlement is also markedly affected, high austenitisation temperatures being detrimental in all respects. Phosphorus segregation has been shown to occur during air cooling from tempering and during isothermal ageing, the degree of segregation increasing with austenitisation temperature, resulting in an increase in DBTT and a reduction in USE. Changes in DBTT and USE on isothermal ageing have been attributed to phosphorus segregation in all four composition melts. Microstructures susceptible to embrittlement have also shown enhanced levels of boron or boron-containing particles at prior austenite grain boundaries.     

The cooperative relation between temper embrittlement and hydrogen embrittlement in a high strength steel

A sample plate of HY 130 steel (5 pet Ni-0.5 pet Cr-0.5 pet Mo-0.1 pet V-0.1 pet C) was found to be quite susceptible to temper embrittlement. Step-cooling produced a shift in transition temperature of 583 K (310°C). In the step-cooled condition the plane strain stress intensity threshold for crack growth in 0.1 N H₂SO₂ was about 22 MNm^-3/2 (20 ksi √in. ) and the fracture mode was inter granular, whereas in the unembrittled condition the threshold for a 1.27 cm (1/2 in.) plate (not fully plane strain) was around 104.5 MNm^-3/2 (95 ksi Vin. ) and the fracture mode was mixed cleavage and microvoid coalescence. The interaction between the impurity-induced and the hydrogen embrittlement is discussed in terms of Oriani’s theory of hydrogen embrittlement. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania     

Temper embrittlement in a Ni-Cr steel containing phosphorus as impurity

Metallurgical and Materials Transactions B - Phosphorus segregates to prior austenite grain boundaries in ferrite during embrittling treatments (below 1000°F) and contributes to temper...     

An AES study of temper embrittlement in a low alloy steel

In situ tensile tests were conducted in an Auger spectrometer on specimens of a low alloy steel susceptible to temper embrittlement. The specimens were tested in ultra high vacuum and in a low pressure of high purity hydrogen. Comparisons were made of the stress-strain curves, the fracture modes and the Auger electron spectra obtained from the fracture surfaces. Specimens were tested in the as-quenched martensitic (quenched from 950°C). the unembrittled (tempered at 650°C) and the embrittled (tempered at 500°C) states. Testing in hydrogen changed the mode of fracture, in the asquenched martensitic material, from a normal dimpled rupture to an intergranular, dimpled rupture mode. AES analysis of the intergranular facets showed, with the possible exception of N. no segregation of alloying or embrittling elements. Hydrogen had no major influence on the mode of fracture of samples in the unembrittled or embrittled states. In the temper embrittled material, grain boundary segregation of Sn. P and N together with enhancement of Ni was observed. In all conditions testing in hydrogen had a marked influence in reducing the strain to fracture.Specimens water-quenched from the austenite region were susceptible to intergranular quench cracking. No segregation of impurities was detected on the prior austenite boundaries and it is concluded that the phenomenon of quench cracking is not associated with such segregation.     

临界热处理对中碳 Q&P 钢组织与性能的影响

摘要：对中碳钢采用Q&P（淬火 碳分配）和I&QP（临界热处理,淬火 碳分配）热处理工艺,通过对试样的显微组织,残余奥氏体的体积分数及其碳含量,硬度及其拉伸性能进行分析,研究了临界加热对中碳Q&P钢组织和性能的影响。实验结果表明,经临界热处理的Q&P钢组织中,除了马氏体和残余奥氏体,还存在部分铁素体,同时残余奥氏体的体积分数较少,马氏体板条更加细小。在相同的碳分配时间下,I&QP试样的硬度和抗拉强度都比Q&P试样低,但由于I&QP试样中软相铁素体的存在以及残余奥氏体能发挥更好的TRIP效应,使得临界热处理的实验钢的伸长率更高,加工硬化指数增加,强塑积更大。     

临界区热处理工艺对X80大变形管线钢力学性能的影响

采用拉伸、夏比冲击、落锤撕裂(DWTT)及SEM等试验方法,研究了临界区热处理工艺生产的X80大变形管线钢微观组织和力学性能.研究结果表明:采用800～860℃保温15 min淬火后,强度可达到X80大变形管线钢要求,并具有优异的变形性能和极佳的低温冲击韧性,-20℃夏比冲击功均值大于240J,夏比冲击剪切面积均值大于90％,-15℃DWTT剪切面积均值大于85％,同时拉伸曲线呈拱顶型.上述工艺参数条件下淬火后,得到铁素体加马氏体双相组织,马氏体体积分数为16.1 ％～18.2％.     

Reversible temper embrittlement of rotor steels

The susceptibility to temper embrittlement of eight different rotor steels has been studied in terms of the effects of composition, of cooling rate from tempering temperature, of isothermal aging, of steel-making practice and of strength level and tempering temperature. The Ni Cr Mo V steels tested showed increasing susceptibility to temper embrittlement with increasing nickel content. The normally marked susceptibility of a high phosphorus 3 pct Cr Mo steel was eliminated by the removal of manganese. Embrittlement in a 3 pct Ni Cr Mo V steel was caused by the equilibrium segregation of solute atoms to the prior austenite grain boundaries. Two Cr Mo V steels tested were not susceptible to temper embrittlement. Electroslag remelting and refining had very little effect on the susceptibility of the steels tested. Strength level and tempering temperature had no effect on the degree of embrittlement of the 3 pct Ni Cr Mo V disc steel. The possibilities of remedial action include an adjustment of the post tempering cooling rate, to optimize the conflicting interests of minimum temper embrittlement and adequate stress relief, and the production of very low manganese rotor steels.     

The effects of tempering reactions on temper embrittlement of alloy steels

The effects of tempering reactions which produce molybdenum-rich carbides on the temper embrittlement of NiCrMo, NiCrMoV, CrMo, and CrMoV steels, particularly embrittlement due to phosphorus segregation, are reviewed. Molybdenum can act as an effective scavenger for phosphorus and other embrittling impurities, but the scavenging is lost when the molybdenum is precipitated in carbides as a result of continued tempering during service at elevated temperatures. This leads to very slow embrittlement, controlled by the rates of alloy carbide formation, rather than by the diffusion of phosphorus, for example. The presence of vanadium apparently retards the embrittlement process even more by interfering with the formation of the molybdenum-rich carbides. Observations of the temper embrittlement behavior, and of the effects of service exposure, in three CrMoV steam turbine rotors are also reported and are shown to be consistent with the previous results.     

Effect of microstructure on the temper embrittlement of Cr-Mo-V steels

The effect of transformation product on the temper embrittlement susceptibility of a Cr-Mo-V steel doped with P and Sn has been investigated at different strength levels. Results show that at low strength levels (< 10R _C ) embrittlement susceptibilities of tempered bainite and ferrite-pearlite structures are comparable to each other, but lower relative to tempered martensite. The lower susceptibility of tempered bainite relative to tempered martensite obtains up to about 40R _C , above which the susceptibilities for the two structures are similar. Variation of embrittlement susceptibility with microstructure is completely consistent with the degree of grain boundary segregation of P and Sn. The segregation is smaller in bainite than in martensite at a given strength level and increases with increasing strength level for a given structure. When compared at the same degree of embrittlement (i.e., same shift in FATT), the amount of grain boundary segregate and the extent of intergranular fracture are lower in tempered bainite compared to martensite indicating that embrittlement of interfaces other than prior austenite boundaries might be an important factor in the embrittlement of bainite structures.     

The influence of microstructure on the susceptibility of titanium alloys to internal hydrogen embrittlement

Beta-processed near-alpha titanium alloys with a large colony microstructure were found to be susceptible to internal hydrogen embrittlement under conditions of sustained loading or fatigue cycling with a dwell period at peak load. The embrittlement occurs by localized increases in hydrogen content at the tips of long, blocked shear bands developed during time-dependent plastic deformation. The key microstructural features responsible for the embrittlement process have been determined to be a large transformed beta colony size and a fine, discontinuous distribution of beta phase within the colony. Alpha-beta alloys that contain thick, continuous beta platelets were determined to be immune to embrittlement. The results are consistent with a previously-proposed model for the embrittlement process.