Influence of Prior Austenite Grain Size on the Degree of Temper Embrittlement in Cr-Mo Steel

Reversible temper embrittlement has been frequently observed in many different low alloy steels serving at high temperature, e.g. order of 500 °C. This type of embrittlement can change the brittle transgranular fracture mode to intergranular decohesion with subsequent change in fracture stress and fracture toughness. The present paper deals with the influence of the prior austenite grain size and isothermal aging time on the degree of embrittlement of 2.25Cr-1Mo steel, which is very popular for its use in power generating and other petrochemical industries. In this research work, the specimens of 2.25Cr-1Mo steel were treated in three different austenitizing temperatures along with different isothermal embrittling time periods. Then the induced degree of embrittlement was characterized by the fracture stress values at −196 °C and area fraction of intergranular failure. The outcome of the experimental results shows that the increase in austenite grain size and/or isothermal embrittling time severely weakens the grain boundary cohesive strength leading to brittle intergranular failures and thus to a greater degree of temper embrittlement.     

Intergranular fracture on fatigue fracture surface of 2.25Cr-1Mo steel at room temperature in air

Low-alloy steels serving for a long time at high temperature (∼500 °C) are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in the fracture morphology of low-alloy steels. The present work describes the effects of heat treatments on impurity element segregation and its subsequent effects on fatigue fracture behavior of 2.25Cr-1Mo steel under different environmental conditions and temperatures. It has been found that either prior impurity element segregation caused during the heat treatment or hydrogen-induced embrittlement due to the presence of water vapor in laboratory air alone cannot produce intergranular fracture on the fatigue surfaces of 2.25Cr-1Mo steel at room temperature in air. The occurrence of intergranular fracture on the fatigue surfaces results from the combined effect of impurity element segregation-induced grain boundary embrittlement and hydrogen-induced embrittlement, and that the proportion of intergranular fracture is a function of prior impurity element segregation provided that the grain boundary segregation level exceeds a certain critical value.     

Critical level of intergranular fracture to affect the toughness of embrittled 2.25Cr-1Mo steels

In general, the low-temperature brittle fracture mode of unembrittled ferritic steel is transgranular cleavage. During temper embrittlement, impurity elements, such as sulfur (S), phosphorus (P), antimony (Sb), arsenic (As), and tin (Sn), segregate to prior austenite grain boundaries, which results in a decrease in the grain boundary cohesive strength. As a result, the brittle transgranular cleavage fracture mode changes to intergranular decohesion in association with the decrease in the critical fracture (stress (σ _F) as well as the fracture toughness (K). However, the appearance of intergranular facets on the fracture surface does not cause a decrease in the K and σ _F values. In this work, quenched and fully tempered 2.25Cr-1Mo steel (in an unembrittled condition that exhibits almost 100% brittle transgranular cleavage fracture) has been embrittled for 24, 96, and 210 h at 520 °C to produce different proportions of intergranular fracture. These unembrittled and embrittled steel specimens were tested to measure K (at −120 and −196 °C) and σ _F (at −196 °C). The experimental results and detailed fractographic observations show that the K and σ _F values decrease with an increase in the area fraction of intergranular fracture, provided that the area fraction of the intergranular facet on the brittle fracture surface exceeded a certain critical level, approximately 20–22%.     

Characterization of a New Fe-C-Mn-Si-Cr Bearing Alloy: Tempered Martensite Embrittlement Susceptibility

Bearing steels containing 1% C and 1.5% Cr have been the usual material of choice for machine components submitted to rolling and contact fatigue, for more than a century. As a rule these steels are quenched from the intercritical gamma + carbide region and tempered at low temperatures (less than 250 °C), in order to retain the high hardness of the martensite matrix and avoid the tempered martensite embrittlement (TME) phenomena, which compromise the toughness of steels tempered at higher temperatures. A new high Si alloy was developed for bearing applications. The inhibiting and/or retarding effect of Si on the kinetics of cementite precipitation leads to a higher temperature of TME occurrence, allowing the tempering of the components at a higher temperature, thus increasing the toughness, without sacrificing the high hardness. The purpose of this work was to confirm the TME resistance of the new alloy. In this work, impact tests result for commercial SAE/AISI 52100 (0.25% Si) and for a modified 52100 containing 1.74% Si, were compared. No evidence of TME was detected on the Si-modified steel.     

Grain Boundary Segregation Behavior in 2.25Cr-1Mo Steel During Reversible Temper Embrittlement

Low alloy steels serving for a long time at high temperature, e.g., around 500 °C, are very sensitive to temper embrittlement due to segregation of various trace elements at prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation in combination with various environmental effects can adversely affect the fracture resistance and fatigue crack propagation rate with subsequent change in fracture morphology of low alloy steels. This article describes the segregation behavior of various elements in 2.25Cr-1Mo pressure vessel steel investigated by AES, FEG-STEM, SEM, and EDS analyses. As confirmed by AES and FEG-STEM, phosphorus is found to be the main embrittling element for isothermal embrittlement. Sulfur and Mo segregation is only evident after longer embrittlement times. In the step-cooling embrittlement, phosphorus is still found to be the main embrittling element, but heavy segregation of sulfur in some isolated intergranular facets was also observed. For P segregation, a Mo-C-P interaction is observed, while sulfur segregation is attributed to site competition between sulfur and carbon atoms.     

Tempered martensite embrittlement in a 32NiCrMoV125 steel

This study focused on tempered martensite embrittlement in a 32NiCrMoV125 steel through examination of the effects of austenite grain size and tempering temperature on the mechanical properties and fracture morphology of this material. Two different austenite grain sizes were obtained by austenitizing at 870 and 950 °C. After quenching, the specimens were tempered in the temperature range of 200–650 °C. The results obtained in this research indicate that by increasing the tempering temperature, the strength and hardness decrease, but ductility increases. However, impact testing indicated that tempered martensite embrittlement occurred when samples were tempered in the range of 250–400 °C. Fractography revealed intergranular and quasi-cleavage fracture. In summary, increasing the austenite grain size decreased strength, but increased impact toughness, except for samples tempered between 200 and 350 °C.     

Influence of Heat Treatments on Microstructure and Toughness of 9%Ni Steel

The 9%Ni low-carbon steel is applied to utilities and processes at temperatures as low as ??196 °C. However, the microstructural features play an important role on the mechanical properties. Notably, the cryogenic toughness and mechanical strength are strongly dependent on the final heat treatment. In this paper, the microstructure of a 9%Ni low-carbon steel was modified by different heat treatments. The hardness and cryogenic toughness were measured and correlated to microstructural features. The material shows a temper embrittlement with intergranular cracking and minimum cryogenic toughness after tempering around 400 °C. Austempering at 480 °C also produced very low toughness results. On the other hand, excellent cryogenic toughness was obtained with single tempering at 600 °C after quenching or normalizing. Even higher toughness was obtained with the double tempering at 670 °C/2 h plus 600 °C/2 h. The amount of reversed austenite and its morphology in the specimen quenched and tempered at 600 °C were shown in the paper.     

Influence of tempering temperature on both the microstructural evolution and elemental distribution in AISI 4340 steels

In the present study, the influence of tempering temperature on the microstructural evolution and prior austenite grain boundary segregation of AISI 4340 steels was investigated by transmission electron microscope and atom probe. The transmission electron microscopy results showed a variation in the microstructure and the morphology of carbides with a change in tempering temperature. Additionally, the chemical compositions of the prior austenite grain boundaries and carbides were quantified by atom probe tomography. An increase in the tempering temperature led to a decrease in the amount of carbon segregated at the prior austenite grain boundary from 7.9 to 1.3 at.%. It was found that a higher tempering temperature can accelerate the diffusion of carbon from the prior austenite grain boundary into carbide. However, phosphorus atoms were segregated mainly at the prior austenite grain boundary in steel tempered at 400°C (up to 0.18 at.%). It was found that formation of film-like carbide and phosphorus segregation along the prior austenite grain boundary is the main cause of embrittlement in steel tempered at 400°C.     

Grain boundary segregation and fracture behavior of boron containing Fe-Mn-Ni-(Mo,w) age hardening alloys

The effects of boron addition on the grain boundary segregation and fracture behavior of tempered Fe-Mn-Ni-Mo and Fe-Ni-Mn-W steels were investigated. High segregation of Mn to prior austenitie grain boundaries resulted in severe grain boundary embrittlement in W-bearing alloys. Boron addition did not significantly affect the grain boundary segregation of other alloying elements. Nevertheless, improvement of tensile properties is observed in 16 ppm boron doped W-bearing steel. Segregation of boron itself to grain boundaries is believed to affect the grain boundary strength of this alloy. Lower Mn segregation in Mo containing steels resulted in the ductile fracture when tempered at 480°C.     

Atom probe investigations on temper embrittlement and reversible temper embrittlement in S 690 steel weld metal

Severe embrittlement was observed in weld material of a brand new penstock of a huge hydro power plant. Temper embrittlement (TE) was found as root case of embrittlement. Reversible temper embrittlement (RTE) treatment characterised by a short-time heating at about 600°C, by which the toughness of embrittled weld material can significantly be recovered, was qualified and successfully applied in the plant. Basic investigations were performed to explain the embrittlement as well as the de-embrittlement effect. By the application of high resolution analytics as Atom Probe Tomography (APT) applied on TE as well as on the RTE-treated material, revealed phosphorus segregation in the grain boundaries as root cause of embrittlement. By application of RTE treatment the APT results revealed, that the phosphorus segregation in the grain boundaries disappeared. The mechanism of this behaviour can be explained by referring the McLean [Grain boundaries in metals. Oxford: Clarendon Press; 1957] based grain boundary equilibrium segregation of phosphorous. During RTE treatment, which occur at higher temperatures (600°C) that segregation (which starts during cooling at about 550°C), desegregation occurs. During this higher temperature, the diffusion is much faster than segregation producing the fast recovery of toughness.     

Metallurgical stability and the fracture behavior of ferritic stainless steels

The influence of aluminum on the thermal stability of ferritic stainless steels has been investigated using two commercial alloys— Armco Type 18SR and AISI430. Two reaction stages have been detected in these alloys during aging at 475 °; each stage is accompanied by changes in the hardness, yield strength, strain-hardening exponent, and elongation to fracture. The initial stage is attributed to the precipitation of carbide and nitride particles and the second stage to the precipitation of the chromium- rich a’ phase. The 430 alloy exhibits more pronounced changes than 18SR during the first stage due to the higher concentration of interstitials retained in solution after quenching. The effects of the second- stage aging reaction are detected after shorter aging times in the 18SR alloy and are more pronounced than in the 430 alloy, consistent with the influence of aluminum on the coherency strains associated with a’ precipitation. The fracture mechanism in both alloys changes from ductile dimples in the solution- treated and quenched condition to a mix of ductile dimples, intergranular fracture, and transgranular cleavage with increased aging times. Longitudinal cracking at the grain boundaries precedes failure of the aged alloys in tension; it is attributed to the combined effects of void initiation at fine grain boundary precipitates, a’ embrittlement that limits localized plasticity, and the transverse stress components resulting from triaxiality after the onset of necking.     

淬回火对10Cr5MoVRE钢组织性能的影响及其第二相析出行为

以10Cr5MoVRE钢为研究对象,对轧态及淬回火(QT)态试样组织性能进行对比研究,利用TEM和能谱仪对QT态试样析出物进行观察分析。力学性能检测结果表明,QT态试样获得了较好的强韧性配比,相较于轧态试样,QT态试样伸长率提高了93.8%,0 ℃条件下的冲击吸收能量提高了6倍以上。借助SEM观察的轧态及QT态10Cr5MoVRE试验钢低温冲击断口形貌,轧态试样的低温冲击断口形貌为脆性解理断裂,QT态的为韧性韧窝断裂。借助OM观察分析,轧态试样组织为粒状贝氏体而QT态为回火索氏体。QT态10Cr5MoVRE钢第二相析出物的TEM观察分析结果表明,碳化物析出相主要是碳化钒、碳化钼及碳化铬形成的复合相,显微夹杂物析出相则是Al₂O₃、MnS、Ca₃(PO₄)₂及稀土化合物等组成的复合夹杂物。     

ANTIMONY GRAIN BOUNDARY SEGREGATION AND ITS SUPPRESSION BY CERIUM IN Fe—2Mn—Sb STRUCTURAL STEELS

Antimony grain boundary segregation in Fe-2%Mn-Sb structure steels has been studied through measurements of the ductile-brittle transition temperature in conjunction with scanning electron microscopy, Auger electron spectroscopy and secondary ion mass spectroscopy. The research result reveals that during tempering or ageing after quenching at 980℃, Sb segregates to grain boundaries with both equilibrium and non-equilibrium natures and brings about temper embrittlement in the steels. Cerium can relieve temper embrittlement of the steels and its segregation to grain boundaries may play an important role in reducing this embrittlement.     

Effect of hydrogen on the properties and fracture characteristics of TRIP 800 steels

The paper describes effect of hydrogen on the properties and fracture characteristics of two variants of TRIP 800 C–Mn–Si steels. The effect of hydrogen was studied by means of tensile tests on specimens previously charged by hydrogen. Hydrogen provoked embrittlement in both variants but only for very high hydrogen content. Hydrogen embrittlement manifested itself mainly by a loss of plasticity. Both steel variants were able to absorb a large amount of hydrogen, up to 50 ppm. Concerning fractographic characteristics, steels containing higher hydrogen content displayed transgranular cleavage fracture. In exceptional cases, an irreversible embrittlement was revealed initiating on non-metallic inclusions.     

Microstructural Characteristics and Mechanical Properties of Low-Alloy,Medium-Carbon Steels After Multiple Tempering

The microstructure and mechanical properties of NiCrMoV-and NiCrSi-alloyed medium-carbon steels were investigated after multiple tempering. After austenitising, the steels were hardened by oil quenching and subsequently double or triple tempered at temperatures from 250 to 500 °C. The samples were characterised using scanning electron microscopy and X-ray diffraction, while the mechanical properties were evaluated by Vickers hardness testing, V-notched Charpy impact testing and tensile testing. The results showed that the retained austenite was stable up to 400 °C and the applied multiple tempering below this temperature did not lead to a complete decomposition of retained austenite in both steels. It was also found that the microstructure, hardness and impact toughness varied mainly as a function of tempering temperature,regardless of the number of tempering stages. Moreover, the impact toughness of NiCrMoV steel was rather similar after single/triple tempering at different temperatures, while NiCrSi steel exhibited tempered martensite embrittlement after single/double tempering at 400 °C. The observed difference was mainly attributed to the effect of precipitation behaviour due to the effect of alloying additions in the studied steels.     

Improved impact toughness of 13Cr martensitic stainless steel hardened by laser

The impact toughness of AISI 403 martensitic stainless steel plate and laser-hardened specimens tempered at various temperatures were examined. Phosphorus was the primary residual impurity responsible for tempered embrittlement of this alloy. The experimental result also indicated that AISI 403 stainless steel was very sensitive to reverse-temper embrittlement. The improved impact toughness of the laser-hardened specimen was attributed to the refined microstructure in the laser-hardened zone.     

Vermeidung der Wasserstoffversprödung vergüteter Stähle bei galvanotechnischen Prozessen durch thermische Legierungsbildung

Avoidance of hydrogen embrittlement of high strength steels during electroplating processes by thermal alloying Low alloyed high strength steels are often electroplated by metal layers protecting against corrosion. For ultra high strength, quenched and tempered steels with yield strengths > 1000 Nmm^?2 embrittlement by hydrogen being envolved during the electrochemical pretreatment as well as metal deposition has to be avoided. More over the corrosion protecting layers should form a diffusion barrier for hydrogen which can be formed during corrosion processes under special circumstances. In this paper two problem solutions including thermal alloying processes will be discussed. Plating the steel substrate with a nickel layer subsequently annealed at a temperature above 800°C in an inert gas atmosphere an austenitic iron-nickel-alloy at the boundary is formed, being a high efficient diffusion barrier for hydrogen. Further zinc plating is improving the corrosion resistance avoiding at the same time pitting corrosion problems. Plating the steel substrate with a copper and a following nickel layer on top and annealing it at the temperature of 800°C a highly corrosion resistant copper-nickel-alloy is formed showing excellent barrier behaviour for hydrogen diffusion. In both cases hydrogen being formed during the plating process itself and penetrating into the base metal does not lead to embrittlement as it is effusing during the annealing procedure.     

GD钢组织及低温冲击韧性研究

分析了900℃淬火及200℃回火后GD钢的显微组织、硬度及低温冲击的断口形貌,研究结果表明:900℃淬火后GD钢组织由粗针状马氏体、残余奥氏体、碳化物组成,200℃回火时,马氏体中析出部分碳化物,回火组织由回火马氏体和碳化物组成。900℃淬火+200℃回火后的GD钢冲击时,随着温度的降低,其冲击功随之减小,随着GD钢所处的环境温度不断升高,断口宏观形貌中反映起裂区和裂纹纤维扩展区所占比例越来越大,微观形貌中存在解理面、撕裂棱和韧窝,其断裂机理为准解理断裂。     

Effect of Nanoscale Cu-Riched Clusters on Strength and Impact Toughness in a Tempered Cu-Bearing HSLA Steel

The effect of Cu-riched clusters on strength and impact toughness in a tempered Cu-bearing high-strength low-alloy (HSLA) steel is investigated. With increasing the tempering temperature, it is found that the yield strength increases firstly, achieving the maximum value (~ 1053 MPa) at the tempering temperature of 450 °C, and then decreases significantly with the rise of tempering temperature. The tempering temperature-dependent yield strength is closely related to the precipitation of Cu-riched clusters. When tempering at 450 °C, the peak strength will be reached as the nanoscale Cu-riched clusters with small size and high number density will cause a strong precipitation strengthening (~ 492 MPa) due to the dislocation shearing mechanism. However, the Cu-riched clusters will coarsen with further increasing tempering temperature, resulting in obvious decrement of yield strength owing to the dislocation bypassing mechanism. Compared with the yield strength, the variation in impact energy displays an inverse tendency and the impact energy is only 7 J for the sample tempered at 450 °C. The fracture mode can be well explained by the competition between the cleavage fracture strength (σ_F) and “yield strength” (σ_Y). Although transgranular cleavage fracture can be found in samples tempered at 450 and 550 °C, the crack propagation along the lath boundaries is prevented in the sample tempered at 550 °C. The reason is that the number density of Cu-riched clusters at lath boundaries decreases and the segregation of Mo element at the lath boundaries is induced, which will increase the bonding energy.     

Effects of Laser Quenching on Impact Toughness and Fracture Morphologies of 40CrNiMo High Strength Steel

The surface of 40CrNiMo steel was quenched with a CO₂ laser, Charpy impact test was conducted at temperatures of 20, 0, and ?20 °C, and the impact absorption energies were measured. The fracture morphologies were observed with SEM, and the influence of microhardness, residual stress, and retained austenite on mechanical behavior of impact fracture after laser quenching was discussed. The results show that the hardened layer depth is more than 1 mm after laser quenching, and hardness is about 480-500 HV. The fracture morphology of the sample is dimple rupture at a temperature of 20 °C; with the lower temperature the fracture dimples become smaller. At a temperature of ?20 °C, the fracture morphologies change from ductile to brittle, which is mainly cleavage fracture. The increase in surface hardness, production of compressive residual stress, and existence of retained austenite after laser quenching are the main mechanisms of increasing impact toughness.