Carbide- matrix interface mechanism of stress corrosion cracking behavior of high-strength CrMo steels

The effects of tempering temperature and carbon content on the stress corrosion cracking (SCC) behavior of high-strength CrMo steels in 3.5 pct NaCl aqueous solution have been studied by means of Auger electron spectroscopy (AES) and scanning and transmission electron micros- copy (SEM and TEM). Experimental results show that the specimens with higher carbon content and tempered at lower temperatures have a higher tendency for intergranular fracture and lower threshold stress intensity K_ISCC The SCC behavior is significantly affected by the distribution of carbide particles, especially carbide coverage on prior austenitic grain boundaries, through a carbide-matrix interface mechanism as the interface is the preferential site for the nucleation and propagation of microcracks because of its strong ability to trap hydrogen atoms. In low- temperature tempered states, there is the serious segregation of carbon in the form of carbide particles at prior austenitic grain boundaries, causing low-stress intergranular fracture. After tempering at high temperatures (≥400 °C), both the coalescence of the carbide particles at the grain boundaries and the increase of carbide precipitation within grains cause the decrease of the tendency for intergranular fracture and the rise of K_ISCC. The higher the carbon content in steels, the more the carbide particles at the grain boundaries and, subsequently, the higher the tendency for low-stress intergranular fracture. The carbide effect on K_ISCC makes an important contribution to the phenomenon that K_ISCC decreases with the rise of yield strength of the steels.     

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.     

Retained Austenite and Tempered Martensite Embrittlement in Medium Carbon Steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Some in situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M₃C films upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe₃C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The situation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which occurs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can occur in the same steels, depending on the tempering conditions.     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Retained austenite and tempered martensite embrittlement in medium carbon steels

Electron microscopy, diffraction and microanalysis, X-ray diffraction, and auger spectroscopy have been used to study quenched and quenched and tempered 0.3 pct carbon low alloy steels. Somein situ fracture studies were also carried out in a high voltage electron microscope. Tempered martensite embrittlement (TME) is shown to arise primarily as a microstructural constraint associated with decomposition of interlath retained austenite into M₃C filM_s upon tempering in the range of 250 °C to 400 °C. In addition, intralath Widmanstätten Fe₃C forms from epsilon carbide. The fracture is transgranular with respect to prior austenite. The sit11Ation is analogous to that in upper bainite. This TME failure is different from temper embrittlement (TE) which o°Curs at higher tempering temperatures (approximately 500 °C), and is not a microstructural effect but rather due to impurity segregation (principally sulfur in the present work) to prior austenite grain boundaries leading to intergranular fracture along those boundaries. Both failures can o°Cur in the same steels, depending on the tempering conditions.     

Grain boundary composition and associated hydrogen cracking of modified 4130 steels

Earlier work on AISI 4130 steels showed that phosphorus segregation to prior austenite grain boundaries was the primary cause for intergranular fracture of these steels when exposed to hydrogen. Reduction of P segregation to grain boundaries by removing the strong segregation couples of Mn-P and Si-P was expected to increase the hydrogen stress cracking resistance of 4130 type steels. Elimination of Mn and/or Si did reduce the concentration of P at prior austenite grain boundaries, but allowed segregation of S and N which acted in the same manner as P, promoting intergranular hydrogen stress cracking.     

The Effect of Phosphorus Content on the Hydrogen Stress Cracking of High Strength 4130 Steel

A series of four 4130 base steels with various phosphorus concentrations was subjected to cathodic charging to determine the effect of P on hydrogen stress cracking resistance. Static fatigue curves for several different yield strengths were obtained for each alloy. At high yield strengths under applied loads of 60 to 80 pct of the yield, 50 ppm P (bulk concentration) was enough to provide sufficient grain boundary P for an impurity-hydrogen interaction which produced intergranular fracture along prior austenite grain boundaries. Decreasing yield strength and applied stress caused a transition in fracture mode to transgranular while the resistance to hydrogen stress cracking increased with decreasing P. Microhardness measurements of prior austenite grain boundaries were made to establish the role of P. The role of P is not apparently related to its capacity as a strengthening element but more probably as a hydrogen recombination poison. Grain boundary hardness measurements for low temperature tempers (200 °C) appear to be valid while those at 500 °C were not.     

The mechanical stability of austenite and cryogenic toughness of ferritic Fe-Mn-AI Alloys

In an attempt to understand the role of retained austenite on the cryogenic toughness of a ferritic Fe-Mn-AI steel, the mechanical stability of austenite during cold rolling at room temperature and tensile deformation at ambient and liquid nitrogen temperature was investigated, and the microstructure of strain-induced transformation products was observed by transmission electron microscopy (TEM). The volume fraction of austenite increased with increasing tempering time and reached 54 pct after 650 °C, 1-hour tempering and 36 pct after 550 °C, 16-hour tempering. Saturation Charpy impact values at liquid nitrogen temperature were increased with decreasing tempering temperature, from 105 J after 650 °C tempering to 220 J after 550 °C tempering. The room-temperature stability of austenite varied significantly according to the(α + γ) region tempering temperature;i.e., in 650 °C tempered specimens, 80 to 90 pct of austenite were transformed to lath martensite, while in 550 °C tempered specimens, austenite remained untransformed after 50 pct cold reductions. After tensile fracture (35 pct tensile strain) at -196 °C, no retained austenite was observed in 650 °C tempered specimens, while 16 pct of austenite and 6 pct of e-martensite were observed in 550 °C tempered specimens. Considering the high volume fractions and high mechanical stability of austenite, the crack blunting model seems highly applicable for improved cryogenic toughness in 550 °C tempered steel. Other possible toughening mechanisms were also discussed. Formerly Graduate Student, Seoul National University.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M_s but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Retained austenite and tempered martensite embrittlement

The problems of detecting the distribution of small amounts (5 pct or less) of retained austenite films around the martensite in quenched and tempered experimental medium carbon Fe/c/x steels are discussed and electron optical methods of analysis are emphasized. These retained austenite films if stable seem to be beneficial to fracture toughness. It has been found that thermal instability of retained austenite on tempering produces an embrittlement due to its decomposition to interlath films of M₃C carbides. The fractures are thus intergranular with respect to martensite but transgranular with respect to the prior austenite. The temperature at which this occurs depends upon alloy content. The effect is not found in Fe/Mo/C for which no retained austenite is detected after quenching, but is present in all other alloys investigated.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM _S but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

The tensile properties of an Fe-4 pct Mo-0.2 pct C martensite tempered under applied stress

In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure o°Curred in tensile test at 600 °C to 700°C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries. TAKATOSHI OGAWA, formerly with Kyoto University. YOSHIFUMI OHMURA, formerly with Kyoto University. This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

The use of a boron addition to prevent intergranular embrittlement in Fe-12Mn

Fe-12 Mn alloys undergo failure by catastrophic intergranular fracture when tested at low temperature in the as-austenitized condition, a consideration which prevents their use for structural applications at cryogenic temperatures. The present research was undertaken to identify modifications in alloy composition or heat treatment which would suppress this embrittlement. Chemical and microstructural analyses were made on the prior austenite grain boundaries within the alloy in its embrittled state. These studies failed to reveal a chemical or microstructural source for the brittleness, suggesting that intergranular brittleness is inherent to the alloy in the as-austenitized condition. The addition of 0.002 to 0.01 wt pct boron successfully prevented intergranular fracture, leading to a spectacular improvement in the low temperature impact toughness of the alloy. Autoradiographic studies suggest that boron segregates to the austenite grain boundaries during annealing at temperatures near 1000 °C. The cryogenic toughness of a Fe-12Mn-0.002B alloy could be further improved by suitable tempering treatments. However, the alloy embrittled if inappropriate tempering temperatures were used. This temper embrittlement was concom-itant with the dissolution of boron from the prior austenite grain boundaries, which reestablishes the intergranular fracture mode.     

Carbide Evolution in Temper Embrittled NiCrMoV Bainitic Steel

Phosphorus segregation to prior austenite grain boundaries in low alloy steel from exposure to temperatures of 300 to 600°C results in a susceptibility for intergranular fracture referred to as “temper embrittlement”. It has been observed that alloying steel with Mo greatly reduces the phosphorus segregation kinetics. Therefore changes in the ferrite matrix composition from carbide precipitation and evolution involving Mo can influence the segregation phenomenon and fracture properties. This study uses analytical electron microscopy of extraction replicas to characterize the changes in carbide chemistry of a NiCrMoV bainitic steel with 0.25 wt% C that accompany the phosphorus segregation during aging at 480°C for up to 3400 hr. The steel was doped with 0.02 wt% P and tempered at 650°C to two different hardness levels, i.e., two different initial carbide distributions. The amount of grain boundary phosphorus segregation produced by aging at 480°C correlates with the level of molybdenum that remains in solution in the ferritic matrix whereas changes in vanadium and chromium appear to have less influence on the temper embrittlement.     

Chi-Carbide in Tempered High Carbon Martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide (χ), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (θ) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to the χ-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of χ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of χ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest thatχ is the dominant carbide in the tempering temperature range examined.     

Chi-carbide in tempered high carbon martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide(x), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (0) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to theX-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of κ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of κ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest that κ is the dominant carbide in the tempering temperature range examined.     

Effects of Tempering Temperature and Mo/Ni on Microstructures and Properties of Lath Martensitic Wear-Resistant Steels

 The tempering behavior was experimentally studied in lath martensitic wear-resistant steels with various Mo/Ni contents after tempering at different temperatures from 200 to 600 ℃. It is shown that a good combination of hardness (HV) (420-450) and -20 ℃ impact toughness (38-70 J) can be obtained after quenching and tempering at 200-250 ℃. The microstructure at this temperature is lath structure with rod-like and/or flake-like ε-carbide with about 10 nm in width and 100 nm in length in the matrix, and the fracture mechanism is quasi-cleavage fracture combining with ductile fracture. Tempering at temperature from 300 to 400 ℃ results in the primary quasi-cleavage fracture due to the carbide transformation from resolved retained austenite and impurity segregation between laths or blocks. However, when the tempering temperature is higher than 500 ℃, the hardness (HV) is lower than 330 and the fracture mechanism changes to ductile fracture due to the spheroidization and coarsening of cementite. Additions of Mo and Ni have no significant effects on the carbides morphologies at low tempering temperatures, but improve the resistance to softening and embrittling for steels when tempered at above 350 ℃.     

The Tensile Properties of an Fe-4 Pct Mo-0.2 Pct C Martensite Tempered under Applied Stress

In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure occurred in tensile test at 600 °C to 700 °C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P, S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries. Formerly with Kyoto University Formerly with Kyoto University This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Characterization of the Carbon and Retained Austenite Distributions in Martensitic Medium Carbon, High Silicon Steel

The retained austenite content and carbon distribution in martensite were determined as a function of cooling rate and temper temperature in steel that contained 1.31 at. pct C, 3.2 at. pct Si, and 3.2 at. pct noniron metallic elements. Mössbauer spectroscopy, transmission electron microscopy (TEM), transmission synchrotron X-ray diffraction (XRD), and atom probe tomography were used for the microstructural analyses. The retained austenite content was an inverse, linear function of cooling rate between 25 and 560 K/s. The elevated Si content of 3.2 at. pct did not shift the start of austenite decomposition to higher tempering temperatures relative to SAE 4130 steel. The minimum tempering temperature for complete austenite decomposition was significantly higher (>650 °C) than for SAE 4130 steel (～300 °C). The tempering temperatures for the precipitation of transition carbides and cementite were significantly higher (>400 °C) than for carbon steels (100 °C to 200 °C and 200 °C to 350 °C), respectively. Approximately 90 pct of the carbon atoms were trapped in Cottrell atmospheres in the vicinity of the dislocation cores in dislocation tangles in the martensite matrix after cooling at 560 K/s and aging at 22 °C. The 3.2 at. pct Si content increased the upper temperature limit for stable carbon clusters to above 215 °C. Significant autotempering occurred during cooling at 25 K/s. The proportion of total carbon that segregated to the interlath austenite films decreased from 34 to 8 pct as the cooling rate increased from 25 to 560 K/s. Developing a model for the transfer of carbon from martensite to austenite during quenching should provide a means for calculating the retained austenite. The maximum carbon content in the austenite films was 6 to 7 at. pct, both in specimens cooled at 560 K/s and at 25 K/s. Approximately 6 to 7 at. pct carbon was sufficient to arrest the transformation of austenite to martensite. The chemical potential of carbon is the same in martensite that contains 0.5 to 1.0 at. pct carbon and in austenite that contains 6 to 7 at. pct carbon. There was no segregation of any substitutional elements.     

Effects of silicon additions and retained austenite on stress corrosion cracking in ultrahigh strength steels

A study has been made of the effects of silicon additions and of retained austenite on the stress-corrosion cracking (SCC) behavior of commercial ultrahigh strength steels (AISI 4340 and 300-M) tested in aqueous solutions. By comparing quenched and tempered structures of 4340 and 300-M i) at equivalent strength and ii) at their respective optimum and commercially-used heat-treated conditions, the beneficial role of silicon addition on SCC re-sistance is seen in decreased Region II growth rates, with no change in K’_ISCC. The beneficial role of retained austenite is demonstrated by comparing isothermally transformed 300-M, containing 12 pct austenite, with conventionally quenched and tempered structures of 300-M and 4340, containing less than 2 pct austenite, at identical yield strength levels. Here, the isothermally transformed structure shows an order of magnitude lower Region II SCC growth rates than quenched and tempered 300-M and nearly two orders of magnitude lower Region II growth rates than 4340, K ISCC values remaining largely unchanged. The results are discussed in terms of hydrogen embrittlement mechanisms for SCC in martensitic high strength steels in the light of the individual roles of hydrogen diffusivity and carbide type.