In situ observation of recovery of lath structure in 9% chromium creep resistant steel

Abstract

The recovery process of the lath structure during tempering was investigated by in situ observations of 9% chromium creep resistant steel using TEM. A decrease of dislocation density inside the lath grains and an increase in lath width takes place during tempering. In situ observations revealed lath boundary migration and the disappearance of laths during tempering. Lath boundary migration is caused by repeat bulging and migration of local parts of the lath boundaries. Relatively small laths tend to disappear, leading to a decrease in the total energy of lath boundaries. All laths do not start to disappear at the same time; the rate of lath boundary migration differs with boundaries. The driving force of these microstructural changes is the strain accumulated by the martensitic transformation. Moreover, the recovery of the lath structure does not occur homogeneously during tempering because the strain does not distribute homogeneously in the lath structure.     

Study on dislocation density,microstructure, and mechanical properties of P92 steel for different heat treatment conditions

The impact of various heat treatment procedures on microstructure, dislocation density, hardness, tensile characteristics, and impact toughness of P92 steel was examined in the current experiment. The martensitic microstructure and average microhardness of 463 HV 0.2±8 HV 0.2 of the normalized steel were prevalent. A tempering procedure was carried out at 760 °C for a range of 2 hours to 6 hours. Additionally, an X-ray diffraction examination was carried out, and the results were used to determine the dislocation density. The normalized sample was characterized by a high dislocation density. The dislocation density was decreased by tempering of normalized samples. With an increase in tempering time, the effect of the treatment coarsened the grains, precipitates, and decreased the area fraction of precipitates. After tempering, MX, M₂₃C₆, and M₇C₃ types precipitates were found to have precipitated, according to energy dispersive spectroscopy and x-ray diffraction research. The ideal tempering period was determined to be 4 hours at a tempering temperature of 760 °C based on the microstructure and mechanical characteristics. Steel that was tempered at 760 °C for 4 hours had a yield strength of 472 MPa, an ultimate tensile strength of 668.02 MPa, and an elongation of 26.05 %, respectively.     

Hydrogen-induced embrittlement of nickel-chromium-molybdenum containing HSLA steels

ABSTRACT

Several advanced nickel-chromium-molybdenum high strength lowalloy steels newly developed by our research team exhibit excellent mechanical strength, toughness and hardenability. However, the phenomenon of hydrogen-induced embrittlement will easily occur for these high strength steels. In this research, the hydrogeninduced embrittlement of 8625-Modified steel (8625M steel) was studied. Experimental results show that the dominant hydrogen trapping site of the 8625M steel is dislocation, of which trapping energy is about 20 kJ/mol, indicating that the hydrogens trapped in the dislocations are diffusible. The as-quenched 8625M steel has the highest dislocation density and accordingly the highesthydrogen content after hydrogen charging. This makes the asquenched 8625M steel exhibit severe hydrogen embrittlement. After tempering at 200°C and 300°C, the dislocation density drops, and hence these tempered specimens have lower ultimate tensile strength loss. After 400°C tempering, the hydrogen embrittlement phenomenon becomes serious again, being ascribed to the formation of needlelike and film-like cementite which will weaken the strength of martensite. After 500°C tempering, the 8625M steel has the lowest dislocation density, and the inter-lath cementite become discontinuous and spheroidal, making the 500°C tempered specimen have the lowest ultimate tensile strength loss and the highest elongation after hydrogen charging in this study.     

Microstructure–property relationships in tempered low alloy Cr–Mo–3·5Ni–V steel

Abstract

The effect of varying normalising and hardening temperatures, before tempering at ~620°C, on the strength and toughness of a low alloy Cr–Mo–3·5Ni–V (wt-%) steel has been examined. Microstructural features including martensite packet and lath size, dislocation density, and precipitate size were measured and used in a Hall–Petch analysis of the strengthening components. It was found that a rms summation of the strengthening contributions to the 0·2% proof stress gave values in good agreement with experimental results. The 50% fracture appearance transition temperature could be described by a relationship involving the fracture facet size and the strengthening contributions from dislocations and precipitates.

MST/1802     

低碳微合金直接淬火钢的组织与力学性能

为了提高低碳直接淬火钢的强韧性能,对一种低碳Nb-V微合金钢进行了轧后直接淬火(DQ)和再加热淬火(RQ)热处理实验,分析了低碳直接淬火钢的的强韧化机理.采用光学显微镜、透射电子显微镜、硬度计、拉伸试验机以及冲击试验机研究了轧后热处理工艺对低碳Nb-V微合金钢组织和力学性能的影响.结果表明,DQ工艺钢马氏体板条间距细小,含有较多的位错亚结构,因此具有较高的强度和韧性.DQ工艺钢马氏体中的大量位错,促进了碳化物弥散析出,产生了显著的二次硬化效果.由于基体中固溶的Nb、V等元素推迟淬火马氏体在回火过程中的各种转变,以及回火时析出的细小弥散碳化物抑制马氏体铁素体回复、再结晶过程,DQ工艺钢表现出较高的回火稳定性.     

Analysis of the microstructure near the crack tip of ASME Gr.92 steel after creep crack growth

The microstructures near to and remote from the tip of a crack in ASME Gr.92 steel were investigated after creep crack growth at 873 and 898 K, focusing on the martensitic lath, the dislocation structure, and precipitates. After creep, the mean lath width near the crack tip was obviously larger than that of the virgin material, whereas the lath width remote from the crack tip was only slightly larger than that of the virgin material. The mean dislocation density near the crack tip markedly decreased after creep, whereas only a small change was observed in the dislocation density remote from the crack tip. The mean size of M₂₃C₆ particles near the crack tip after creep was larger than that of the virgin material, whereas their mean size remote from the crack tip was almost the same as that of the virgin material.     

Rapid thermomechanical tempering of iron–carbon martensite

Abstract

Tempering of martensite under simultaneous compressive stress has been studied within the temperature range of 20–400°C. Resistive heating was utilised to obtain rapid heating and cooling cycles of a few seconds. Material was obtained from a medium carbon pearlitic railway wheel steel, quench hardened to obtain martensitic structure. Greater than ～150°C dilatation effects where observed below the global yielding point of the material. Microstraining around dislocations in the body centred tetragonal crystallographic structure or viscous flow at higher temperatures was a probable explanation to this material behaviour. Hence, external stress may have an important influence on the tempering progression of martensitic steel. The trials also showed that tempering of martensite progresses fast, is near instantaneous and is independent of the presence of external stress or not.     

Effect of austenitising temperature on microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel

Abstract

The microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel were investigated after austenitising over a range of temperature between 1133 and 1483 K. The experimental results show that the isotropy of impact toughness and mechanical properties were greatly improved due to the disappearance of undissolved aligned second phase when the austenitising temperature was over 1233 K. When the austenitising temperature was over 1383 K, martensite lath and packet abnormally grew up due to dissolution of spheroidal Nb rich carbonitrides; both the platelet size and morphology of martensite were changed, which has an effect on the mechanical properties of the samples. It was noticed that the finer self-tempered carbides, which strengthened martensitic matrix, appeared after austenitising temperature over 1283 K. The strength profiles show a marked plateau for the samples austenitised from 1283 to 1433 K; however, the strength was deteriorated due to coarsening of these self-tempered carbides at 1483 K. It was confirmed that calcium treatment can help improve the isotropy of mechanical properties by modifying sulphide inclusion morphology.     

Surface carburised AISI 8620 steel with dual phase core microstructure

Abstract

In this study, the production of dual phase steel structure in the core of surface carburised AISI 8620 cementation steel and the effect of martensite volume fraction on tensile properties have been investigated. For these purposes, surface carburised (~0·8 wt-%C) specimens were oil quenched from 900°C to obtain a fully martensitic starting microstructure. Then specimens were oil quenched from intercritical annealing temperatures of 731 or 746°C to produce dual phase steel structure in the core of specimens with martensite fractions of ~25 or ~50 vol.-% and nearly wholly martensitic microstructure at the surface. Generally, specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strengths but superior ductility without sacrificing surface hardness than those specimens with fully martensitic microstructure in the core produced by using conventional heat treatment involving quenching from 850 to 950°C. Also tensile strength increased and ductility decreased with increasing martensite volume fraction.     

Cr含量降低对H13钢组织与力学性能的影响

对比了Cr含量降低为3%的3Cr-H13钢与Cr含量为5%的传统H13钢性能的差异,利用SEM、TEM、XRD进行微观组织与相组成分析,研究了Cr对H13钢组织性能的影响。结果表明,Cr含量的降低明显提高了H13钢的回火稳定性与高温强度,其原因主要与回火组织中马氏体的回复程度及二次析出碳化物的种类有关。传统H13钢在650℃回火时,马氏体基本回复完全,基体强度明显下降,并在原马氏体板条界和晶界上析出了较多的尺寸为120nm左右的近球形Cr7C3和M6C型碳化物,第二相强化效果降低;而Cr含量降低为3%的3Cr-H13钢在650℃回火后,基体依然为板条马氏体,板条内保持较高的位错密度,同时板条内析出的大量细小弥散的短棒状VC,在起到弥散强化作用的同时还钉扎位错,推迟了马氏体的回复,从而提高了高温性能。     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 1 – Microstructural analysis

Abstract

Specimens of 16Cr - 5Ni - 1Mo stainless steel were solution treated at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, optical microscopy, scanning (SEM) and transmission (TEM) electron microscopy, and X-ray diffraction examinations were conducted. The microstructure of all aged specimens was found to consist of martensite with variable fractions of δ ferrite and reversed austenite. Very fine precipitates of Mo carbides were revealed in the specimens aged at 475 ° C. The specimens aged at 625 ° C showed a decrease in the dislocation density and a high volume fraction of austenite and precipitation of Fe₂Mo Laves phase was detected by X-ray analysis. Above 625 ° C, Cr₂₃C₆ and TiC became the predominate carbides heterogeneously precipitated in the martensitic matrix. Partial transformation of reversed austenite to unaged martensite was observed at temperatures above 625 ° C.     

Effect of tempering temperature on monotonic and cyclic properties of a high–strength low–alloy chain steel

Abstract

The influence of tempering temperature in the range 200–600°C on the monotonic and cyclic response of a high-strength, low–alloy steel used for haulage chains has been investigated. Static properties are little affected by tempering up to 400°C, with a small loss of strength arising from carbide coarsening. At higher tempering temperatures, dislocation annihilation is substantial and the strength falls markedly. On undergoing high–strain fatigue, the alloy exhibits cyclic softening at all tempering temperatures, but with a maximum of about 25% after a 400°C temper. The microstructural changes producing this are difficult to detect directly, but it is thought that dislocation unpinning has an important role. For tempering temperatures in excess of 400°C, the reduced softening can clearly be equated with a dislocation redistribution and the formation of cell substructure within the ferrite grains. Estimations of the magnitude of the various components of strength are made.

MST/199     

Studies on influence of reheating temperature and cooling rate on structure–property behaviour of lean chemistry HSLA-100 steel

Abstract

Simulation studies on the influence of reheating temperature on austenite grain coarsening in lean chemistry high strength low alloy (HSLA)-100 steel were carried out to establish optimum soaking temperature before hot rolling. Experiments carried out in ‘Gleeble-3500’ dynamic thermomechanical simulator revealed that prior austenite grain sizes varied between 26 and 98 and 34 and 126 μm after soaking at 1150, 1200 and 1250°C for 1 and 5 min respectively; a soaking temperature of 1200°C was found to be optimum. Simulation experiments on the influence of cooling rate on microstructural changes and dilatometric studies indicated lowering of transformation temperature with faster cooling. Microstructural examination of dilatometric samples confirmed martensitic transformation at faster cooling rate. The martensite structure is desirable to achieve better strength and toughness. The findings of simulation studies were subsequently used for standardising thermomechanical treatments of Nb–Cu bearing lean chemistry HSLA-100 steels. One laboratory heat of Cu bearing HSLA steel containing 0·028%Nb was made. This heat was hot rolled into 12·5 mm thick plate by varying finish rolling temperature in the range of 800–1000°C. The soaking temperature was maintained at 1200°C. The rolled plates were heat treated by both conventional reheat quenching and tempering (RQT) as well as direct quenching and tempering (DQT) techniques. Evaluation of mechanical properties revealed that plates processed through DQT route were superior to those processed through RQT route. Transmission electron microscopy revealed that martensite structure and finer interlath spacing in DQT plates resulted in superior strength and impact toughness properties as compared to RQT steels.     

Thermal aging of 16Cr – 5Ni – 1Mo stainless steel Part 2 – Mechanical property characterisation

Abstract

Mechanical property characterisation has been carried out on specimens of 16Cr - 5Ni - 1Mo stainless steel, subjected to various aging cycles. The heat treatment cycles involved solution treatment at 1050 ° C for 1 h followed by heating in the temperature range 400 - 750 ° C for different holding times (1 - 16 h). After heat treatment, tensile, hardness, impact, and creep tests were conducted. Specimens aged at 475 ° C exhibited maximum values of tensile strength and hardness with minimum values of ductility and impact toughness, while specimens aged at 625 ° C had maximum values of impact toughness and ductility. The results were correlated with the microstructural data presented in Part 1 of this study. Softening of the martensitic matrix at 625 ° C occurs as a result of the elimination of internal stresses, the decrease in the dislocation density, and the high volume fraction of austenite which lead to the drop in values of tensile strength and hardness. The results of the study reveal that aging at 550 ° C for 4 h gives the optimum combination of strength, hardness, ductility and toughness for this steel.     

Microstructure of as-quenched 3.5NiCrMoV rotor steel. Part I. General structure and retained austenite

The microstructural features have been examined for 3.5NiCrMoV steam turbine rotor steel, in the as-quenched state and tempered at 500 °C. Quenching produces lath martensite, with bands of retained austenite at the lath boundaries and, to a lesser extent, at prior austenite grain-boundaries. Autotempering occurs during the quench, resulting in loss of tetragonality of the martensite and extensive carbide precipitation in the matrix and to a lesser degree at prior austenite grain boundaries, but not at lath boundaries. Tempering at 500 °C leaves the lath structure largely intact, but causes retained austenite to transform to bands of ferrite and cementite. This transformation does not correlate with the reduction in stress corrosion crack velocity which occurs on tempering. The strength of 3.5NiCrMoV steel in the as-quenched and 500 °C tempered conditions is most probably due to the combination of carbide precipitation strengthening and substructure strengthening.     

Effect of processing parameters on the evolution of dislocation density and sub-grain size of a 12%Cr heat resistant steel during creep at 650 °C

The influence of hot-deformation and tempering temperature on the microstructure evolution of a 12%Cr heat resistant steel during short-term creep at 80-250 MPa and 650 °C was investigated. Quantitative determination of dislocation density and sub-grain size in the initial microstructure and after creep was investigated by STEM-HAADF. A correlation between microstructure evolution and creep response is established. All crept samples showed a significant increase of sub-grain size and a reduction of dislocation density. Hot-deformed samples showed better creep strength than non hot-deformed samples due to homogenization of the microstructure. The tempering temperature affected the dislocation density and the sub-grain size evolution, influencing the creep behavior.     

Influence of irradiation temperature on microstructure of EU batch of ODS Eurofer97 steel irradiated with neutrons

Abstract

A TEM investigation of an EU batch of oxide dispersed strengthened (ODS) Eurofer97 steel specimens, irradiated to 1 and 3 dpa at 300, 450 and 550°C in high flux reactor at Petten, has been performed to understand the influence of irradiation temperature on the characteristics of irradiation defects and, eventually, on the resulting mechanical properties of this material. Specimens irradiated at 300°C revealed the presence of a high density of black dot damage and small self-interstitial atom (SIA) dislocation loops causing substantial hardening and embrittlement. In contrast, negligible black dot damage, low density of large SIA loops and networks of dislocations are observed in specimens irradiated at 450 and 550°C. The lath martensitic structure and ODS particles remain unaffected after irradiation in all specimens. These results are discussed in view of possible activation of defect annihilation mechanisms to explain the observed recovery of mechanical properties at high irradiation temperatures.     

Phase transformation and mechanical behaviour of thermo-mechanically controlled processed high-strength multiphase steel

A novel low-alloy high-strength steel [Fe–0.20C–1.65Mn–1.40Si–1.50Al–1.30Cu–1.05Ni–1.07Co (wt%)] has been thermo-mechanically processed with a finish rolling temperature of 850 °C, followed by air cooling and water quenching in order to obtain a good combination of strength and ductility. Phase transformations of the above steel at different cooling rates have been studied and continuous cooling transformation (CCT) diagram has been constructed using data, obtained from dilatometric study. The phase field of CCT diagram indicates microstructure changes from a mixture of ferrite and bainite to fully martensite accompanied with the enhancement of hardness with increasing cooling rate. The microstructural investigation at lower cooling rate (≤5 °C/s) suggests the possibility of achieving pearlite-free microstructure by direct air cooling from the austenite region. Directly air-cooled steel has demonstrated primarily ferrite–bainite microstructure, which shows attractive tensile strength (>1050 MPa) and ductility (>15 %). On the other hand, directly water-quenched steels reveal predominantly lath martensitic microstructure with high dislocation density which exhibits higher tensile strength (>1600 MPa) and lower ductility (~12 %). The multiple stages of strain hardening behaviour of the investigated steel under different cooling conditions have been examined with respect to microstructural evolution.     

Effect of weld heat input on toughness and structure of HAZ of a new super-high strength steel

Fracture morphology and fine structure in the heat-affected zone (HAZ) of HQ130 super-high strength steel are studied by means of SEM, TEM and electron diffraction technique. Test results indicated that the structure of HAZ of HQ130 steel was mainly lath martensite (ML), in which there were a lot of dislocations in the sub-structure inside ML lath, the dislocation density was about (0.3∼ 0.9) x 10¹²/cm². No obvious twin was observed in the HAZ under the condition of normal weld heat input. By controlling weld heat input (E ≤20 kJ/cm), the impact toughness in the HAZ can be assured.     

Temper embrittlement of hot work die steel

Abstract

The influence of the parameters of an additional tempering at 600°C on temper embrittlement in quenched and previously double tempered 5 wt-%Cr tool steel having a post-martensitic microstructure was investigated. A detailed examination of associated changes of fracture mechanism, microstructure, and precipitate was carried out. The dominant factor controlling the evolution of brittleness was the segregation of phosphorus to prior grain and lath boundaries as well as to martensite lath/carbide interfaces. The segregation effect of phosphorus at these interfaces was accompanied by cosegregation of sulphur and silicon together with simultaneous carbide precipitation.

MST/1132