Effect of heat treatment on the fracture toughness of hot-rolled corrosion-resistant austenitic high-nitrogen 0.4Cr20Ni6Mn11Mo2N0.5 steel

The effect of water quenching from rolling heating at 1100°C on the structure, the mechanical properties, and the static fracture toughness of corrosion-resistant austenitic high-nitrogen 0.4Cr20Ni6Mn11Mo2N0.5 steel shot-rolled at 1100–900°C is studied. It is found that, after quenching, the initially hot-deformed steel possesses a quite high fracture toughness, although quenching from 1100°C decreases its fracture toughness by 11.4%. An analysis of fracture surfaces indicates a ductile character of failed quenched steel specimens. The specimens have fracture regions close to a quasi-cleavage at the stage of stable crack growth.     

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.     

Effect of Carbon on Stainless Austenitic CrMnCN Steel Castings

The effect of up to 0.5 mass% C on a steel with 18 to 19 mass% Cr and Mn each and about 0.6 mass% N was investigated by tensile tests, notch impact tests and corrosion tests. The experimental results show that the solution anneal temperature and the sensitisation to intercrystalline corrosion depend on the carbon content which raise the strength and cold work hardening. Up to 1 mass% C+N the ductile to brittle transition temperature remains at about ?90 °C. Corrosion in diluted aqueous solutions of H₂SO₄, HCl and NaCl is described.     

748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior

22Cr-5Ni duplex stainless steel (DSS) was aged at 748 K (475 °C) and the microstructure development correlated to changes in mechanical properties and fracture behavior. Tensile testing of aged microstructures confirmed the occurrence of 748 K (475 °C) embrittlement, which was accompanied by an increase of strength and hardness and loss of toughness. Aging caused spinodal decomposition of the ferrite phase, consisting of Cr-enriched α″ and Fe-rich α′ and the formation of a large number of R-phase precipitates, with sizes between 50 and 400 nm. Fracture surface analyses revealed a gradual change of the fracture mode from ductile to brittle delamination fracture, associated with slip incompatibility between ferrite and austenite. Ferrite became highly brittle after 255 hours of aging, mainly due to the presence of precipitates, while austenite was ductile and accommodated most plastic strain. The fracture mechanism as a function of 748 K (475 °C) embrittlement is discussed in light of microstructure development.     

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).     

Thermal embrittlement of 18 Ni(350) maraging steel

The embrittlement of as-solutionized 18 Ni(350) Maraging steel was monitored as a function of heat treatment variables by means of Charpy impact tests. The processing parameters of interest were annealing temperatures in the range of 1900° to 2400°F, intermediate holding temperatures in the range of 1300° to 1800°F, and the quenching rate. The changes in fracture mode with heat treatment were characterized by replica and scanning electron microscopy. The severity of thermal embrittlement increases with decreasing cooling rate from the annealing treatment upon direct quenching to room temperature. Intermediate isothermal holding, particularly at 1500° to 1600°F, further accentuates the embrittlement. A large grain size is beneficial to the toughness when rapid direct quenches from the annealing range are imposed but is detrimental upon air cooling or intermediate holding. The major loss in toughness may be associated with the diffusion of interstitial impurity atoms (C+N) to the austenite grain boundaries during cooling or intermediate isothermal holding below 2000°F. An advanced stage of the embrittlement is characterized by the discrete precipitation of Ti(C,N) platelets on these boundaries. Thermal embrittlement is accompanied by change in fracture mode from transgranular dimpled rupture to intergranular quasi-cleavage.     

常化温度对2.3% Si无取向硅钢2.5 mm热轧板冲击韧性的影响

对900-950℃常化处理后的2.3%Si无取向硅钢进行了室温至100℃的冲击试验,采用扫描电镜观察不同条件下的冲击断口形貌,分析了常化温度对2.3%Si无取向硅钢韧性的影响。结果表明,当冲击试验为室温时,随常化温度的升高,试验钢由韧性断裂逐渐转为脆性断裂,冲击功由21.4J降至12.2J;当冲击温度为100℃时,经900～950℃常化处理后的试验钢冲击功为28.4～33.2J,为韧性断裂;经常化处理后的冲击断口区域夹杂物主要为AlN。     

Impact Toughness of Ultrafine-Grained Interstitial-Free Steel

Impact toughness of an ultrafine-grained (UFG) interstitial-free (IF) steel produced by equal-channel angular extrusion/pressing (ECAE/P) at room temperature was investigated using Charpy impact tests. The UFG IF steel shows an improved combination of strength and impact toughness compared with the corresponding coarse-grained (CG) one. The CG IF steel samples underwent a transition in fracture toughness values with decreasing temperature because of a sudden change in fracture mode from microvoid coalescence (ductile) to cleavage (brittle) fracture. Grain refinement down to the submicron (??320?nm) levels increased the impact energies in the upper shelf and lower shelf regions, and it considerably decreased the ductile-to-brittle transition temperature (DBTT) from 233?K (?40?°C) for the CG steel to approximately 183?K (?90?°C) for the UFG steel. Also, the sudden drop in DBTT with a small transition range for the CG sample changed to a more gradual decrease in energy for the UFG sample. The improvement in toughness after UFG formation was attributed to the combined effects of grain refinement and delamination and/or separation in the heavily deformed microstructure. Although an obvious change from the ductile fracture by dimples to the brittle fracture by cleavage was recognized at 233?K (?40?°C) for the CG steel, no fully brittle fracture occurred even at 103?K (?170?°C) in the UFG steel.     

Improvement of a Set of 13Cr11Ni2W2MoV Steel Mechanical Properties for Critical Components of Stop Valves of Oil and Gas Pipelines Using Intercritical Quenching

Metallurgist - A heat treatment regime for steel 13Cr11Ni2WMoV quenched from 980°C with the aim of simultaneous achievement of the required values of impact toughness and strength is...     

Effects of Microalloying on the Impact Toughness of Ultrahigh-Strength TRIP-Aided Martensitic Steels

The effects of the addition of Cr, Mo, and/or Ni on the Charpy impact toughness of a 0.2 pct C-1.5 pct Si-1.5 pct Mn-0.05 pct Nb transformation-induced plasticity (TRIP)-aided steel with a lath-martensite structure matrix (i.e., a TRIP-aided martensitic steel or TM steel) were investigated with the aim of using the steel in automotive applications. In addition, the relationship between the toughness of the various alloyed steels and their metallurgical characteristics was determined. When Cr, Cr-Mo, or Cr-Mo-Ni was added to the base steel, the TM steel exhibited a high upper-shelf Charpy impact absorbed value that ranged from 100 to 120 J/cm² and a low ductile–brittle fracture appearance transition temperature that ranged from 123 K to 143 K (?150 °C to ?130 °C), while also exhibiting a tensile strength of about 1.5 GPa. This impact toughness of the alloyed steels was far superior to that of conventional martensitic steel and was caused by the presence of (i) a softened wide lath-martensite matrix, which contained only a small amount of carbide and hence had a lower carbon concentration, (ii) a large amount of finely dispersed martensite-retained austenite complex phase, and (iii) a metastable retained austenite phase of 2 to 4 vol pct in the complex phase, which led to plastic relaxation via strain-induced transformation and played an important role in the suppression of the initiation and propagation of voids and/or cleavage cracks.     

Effects of chromium on the temper embrittlement and wear resistance of 0.25% C low-alloy cast steel

The effects of additions of 0.6 to 2.0% Cr on the temper embrittlement behaviour of 0.25 C–1.0 Si–1.3 Mn cast steel under several hardening conditions were studied. The susceptibility to temper embrittlement, transgranular and intergranular fracture were increased as the chromium content increased when the steels were tempered at 350°C and slowly cooled from 550°C. The impact toughness and abrasion resistance of the steels were found to depend to a great extent on the Cr-content and tempering temperature.     

Fracture of high-nitrogen 05Kh20G10N3AMF steel during impact loading

Specimens with V- and U-shaped notches made of austenitic high-nitrogen corrosion-resistant 05Kh20G10N3AMF steel are subjected to impact tests in the temperature range from +20 to ?196°C, and stress-strain diagrams are recorded. The test data are used to estimate impact toughness KCV and KCU, dynamic fracture toughness J _id at the stage of crack nucleation, and crack propagation energy A _p. The microrelief of the fracture surfaces is studied. As compared to forging, quenching from 1100°C is found to increase the impact toughness and the dynamic cracking resistance of the steel during impact loading and to decrease the ductile-brittle transition temperature. The steel is shown to exhibit the cold brittleness behavior characteristic of bcc materials. A model is proposed for the formation of cleavage facets in austenitic steels. It is based on easy slip along lattice planes under the low shear stress at a notch tip and the development of fracture at a stress lower than the yield strength of the material.     

Effect of tempering on mechanical properties of 25Co15Ni11Cr2MoE steel

The change rule of mechanical properties and impact fracture morphologies of a high Co- Ni secondary hardening ultra- high strength 25Co15Ni11Cr2MoE steel tempered at 200-750?? after quenched was studied by mechanical properties test and microstructure analysis such as optical microscope(OM) and scanning electron microscope(SEM). The results show that experimental steel after quenching and tempering has a remarkable secondary hardening effect. After tempered at 400-495??, the hardness of experimental steel can reach and beyond the quenched hardness. In this range, tensile strength, yield strength and hardness of experimental steel increase with the tempering temperature increasing, tensile strength and hardness of experimental reach maximum (57. 3HRC and 2160MPa) after tempered at 470??, meanwhile, with the tempering temperature increasing, impact toughness of experimental steel decreases during the prophase, until reaches minimum at 430??, then increases gradually, and reaches maximum after tempered at 510??. The recommended optimum heat treatment process of 25Co15Ni11Cr2MoE steel is as follow: 950???1h oil quenching??(-73??)??1h rising back to room temperature in the air ??495???5h air cooling. At this time, the experimental steel has the best strength and toughness matching.     

Optimizing structure,toughness and wear performance of 15 % Cr cold work cast tool steel

A series of six Cr-, Cr + Mo-, Cr + Mo + V cold work cast tool steels were produced and investigated for microstructure, impact toughness and both experimental and industrial abrasive wear. Grain refinement of the steel matrix even in as-cast condition was obtained on using 2.3 % Mo + 0.9 % V and that ensured increasing impact toughness and abrasion resistance. An optimum impact toughness of about 85 J-cm^?2 was obtained in air quenched (970°C) and tempered (450°C) Mo + V containing steels in which area fraction of carbides reached 38 %. The abrasion resistance improved in case of steels tempered at 250°C and had fine grain structure.     

The Analysis of Low Temperature Toughness on X80 Pipeline Steel Welded Joint

Aiming at the security problems of pipeline steel application, the different positions of the welded joints of circumferentially welding pipeline of X80 steel were investigated by microstructure observation, the hardness, Charpy impact toughness and crack tip opening displacement (CTOD) test at low temperature. The Vickers hardness test results show that there are local softened regions in heat-affected zone (HAZ). Charpy impact test indicate that the ductile–brittle transition temperature of weld is below ??60 °C, the ductile–brittle transition temperature of HAZ is around ??38 °C. CTOD test reveal that the fracture toughness of HAZ shows a large fluctuation since it is in the ductile–brittle transition temperature regime.     

Mechanical properties of a 29 Pct Cr-4 Pct Mo-2 Pct Ni ferritic stainless steel

The mechanical properties of a new ferritic stainless steel consisting essentially of 29 pct Cr, 4 pct Mo, 2 pct Ni (29-4-2) have been evaluated. The mechanical properties of the alloy are dependent on the thermomechanical processing and the final heat treatment conditionsi.e., both annealing temperature and cooling rate from the anneal. The alloy has excellent toughness, ductility and strength at room temperature when fast cooled from elevated temperatures. Slow cooling from elevated temperatures results in a degradation of impact resistance and an increase in strength. The alloy is subject to two major forms of embrittlement. One form results from the precipitation of intermetallic compounds in the temperature range 704°C (1300°F) to 954°C (1750°F) while the other results from the classical phenomenon called 475°C (885°F) embrittlement in the temperature range 399°C (750°F) to 510°C (950°F). Degradation of room temperature impact resistance occurs faster after the high temperature type of embrittlement and failure is characterized by an intergranular fracture mode. Embrittlement after exposure to 475°C (885°F) results in a slower degradation in toughness and results in failure by a transgranular cleavage mode. Impact resistance and tensile ductility are also decreased by exposure to 593°C (1100°F); however, to a lesser degree than 475°C (885°F) or 760°C (1400°F) exposure. The alloy deforms by slip or twinning depending on the metallurgical condition of the material. Deformation by twinning rather than slip is not manifested by a reduction in either toughness or ductility. Exposure to 482°C (900°F) promotes deformation by twinning whereas exposure to 760°C (1400°F) does not.     

Fracture behavior in medium-carbon martensitic Si- and Ni-steels

A study of fracture behavior in medium-carbon martensitic Ni- and Si-steels has been made. Charpy impact testing was conducted in order to investigate fracture mode as a function of test temperature. Whereas the transgranular cleavage fracture becomes the primary brittle fracture mode with decreasing test temperature in the Si-steel, intergranular fracture is the only brittle fracture mode observed at low temperatures in the Ni-steel. The different fracture behavior between these steels appears to be due to variation in intrinsic matrix toughness. Since Si may impair the intrinsic matrix toughness, the occurrence of transgranular cleavage fracture becomes relatively easy with decreasing test temperature. On the other hand, since Ni may improve considerably the intrinsic matrix toughness, the transgranular cleavage fracture is not able to occur although the test temperature decreases. Thus, the intrinsic matrix toughness can play a significant role in controlling the fracture behavior.     

Embrittlement in CN3MN Grade Superaustenitic Stainless Steels

Superaustenitic stainless steels (SSS) are widely used in extreme environments such as off-shore oil wells, chemical and food processing equipment, and seawater systems due to their excellent corrosion resistance and superior toughness. The design of the corresponding heat treatment process is crucial to create better mechanical properties. In this respect, the short-term annealing behavior of CN3MN grade SSS was investigated by a combined study of Charpy impact tests, hardness measurements, scanning and transmission electron microscopy. Specimens were heat treated at 1200 K (927 °C) for up to 16 minutes annealing time and their impact strengths and hardnesses were tested. The impact toughness was found to decrease to less than the half of the initial values while hardness stayed the same. Detailed fracture surface analyses revealed a ductile to brittle failure transition for relatively short annealing times. Brittle fracture occurred in both intergranular and transgranular modes. SEM and TEM indicated precipitation of nano-sized intermetallics, accounting for the intergranular embrittlement, along the grain boundaries with respect to annealing time. The transgranular fracture originated from linear defects seen to exist within the grains. Close observation of such defects revealed stacking-fault type imperfections, which lead to step-like cracking observed in microlength scales.