Mechanism of embrittlement in tempered martensite

Abstract

In this paper the total driving force for the decomposition of retained austenite and martensite are calculated together with the nucleation and growth characteristics of cementite in the two phases. The results demonstrate that the driving force for the decomposition of martensite is an order of magnitude less than that of austenite. However, the driving force for cementite precipitation in martensite is two orders of magnitude greater than in austenite with a much shorter incubation period. On short term tempering cementite precipitates from martensite whereas on longer term tempering decomposition of retained austenite occurs because of the increase in driving force which is enhanced by the contraction of the martensite on decomposition. It is argued that the precipitation of cementite from the austenite results in tempered martensite embrittlement, a mechanism dependent upon the two related decomposition processes. The segregation of trace impurities or the precipitation of cementite at the grain boundaries is not a prerequisite.

MST/240     

The application of metallographic techniques to the study of the tempering of HSLA-100 steel

A study of a quenched, and a quenched and tempered low-carbon, copper-containing steel (HSLA 100) was conducted. The primary investigative technique employed was transmission electron microscopy. Quenching the steel from the austenitizing temperature yielded a microstructure that was predominantly lath martensite. However, a significant amount of retained austenite was also present. Niobium carbide particles were also documented in the as-quenched structures. Tempering at 605°C for 1–3 h yielded a heterogeneous distribution of ---Cu, much of which was associated with the lath boundaries. The austenite was highly resistant to decomposition during tempering at 605°C and, as a consequence, little evidence of cementite precipitation was found.     

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.     

Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels

Tempering approach is designed for better understanding the effects of heat treatment induced by production process when manufacturing on the transformation-induced plasticity steels containing 1.0 wt.% Al. Specific attention is placed on the roles of tempering temperature and the holding time on the decomposition of retained austenite and the redistribution of alloying elements. Using transmission electron microscopy, we found the retained austenite was decomposed into ε-carbide and ferrite in the steels tempered at 300 °C for 9 h. An increase in the temperature of 400 °C and the holding time over 3 h accelerate the nucleation kinetics of cementite formation, leading to the deteriorated thermal stability of austenite. In addition, atom probe tomography studies confirmed the partitioning tendency of alloying elements across the ferrite/cementite interfaces as well as the compositional spikes of Mn at the interfaces during tempering over 400 °C for 9 h.     

Wear of Laying Head Pipes in a Wire Rod Mill

The failure analysis on the premature wear of a “Laying Head Pipe” in a Wire Rod Mill has been presented. The hot-rolled wire rods subsequent to finish rolling pass through the “Laying Head Pipe” which rotates and lays the wire rods in the form of coils for air cooling to achieve the final properties. A worn-out pipe and a thermo-mechanically treated (TMT) re-bar have been analyzed. The material of the pipe is ASTM A335-P5 grade of seamless alloy steel pipe used for high temperature service. The microstructures of the wear groove of the alloy steel pipe show predominantly ferrite and globular cementite/carbide particles along with scales, while that away from the wear groove shows coarse tempered martensite matrix. EDS analysis confirms the presence of alloy carbides near the wear groove. Microhardness profile shows reduction in hardness toward the inner surface of pipe; hardness at the inner surface of the pipe becomes lower than the surface hardness of the TMT re-bar, exhibiting tempered martensite matrix. Softening at the inner surface of the pipe wall occurs due to a rise in temperature/ over-tempering in contact with the passing hot wire rods (900 °C) which causes transformation of martensite into ferrite and coarse globular cementite.     

Cementite precipitation during tempering of martensite under the influence of an externally applied stress

The precipitation of cementite under the influence of an externally applied stress, during the tempering of martensite in steels, is investigated using transmission electron microscopy. The stress appears to favour the development of particular crystallographic variants of cementite in any given plate of martensite. Hence, a Widmanstätten array of cementite particles in a normally tempered sample changes to an array consisting of just one variant in stress-tempered samples. The results are discussed in the context of the mechanism of carbide precipitation during the lower bainite reaction.     

Mechanical properties of ultrafine grained ferritic steel sheets fabricated by rolling and annealing of duplex microstructure

A new route to fabricate ultrafine grained (UFG) ferritic steel sheets without severe plastic deformation is proposed in this article. A low-carbon steel sheet with a duplex microstructure composed of ferrite and martensite was cold-rolled to a reduction of 91% in thickness, and then annealed at 620–700 °C. The microstructure obtained through the process with annealing temperatures below 700 °C was the UFG ferrite including fine cementite particles homogenously dispersed. The grain size of ferrite matrix changed from 0.49 to 1.0 μm depending on the annealing temperature. Dynamic tensile properties of the produced UFG steels were investigated. The obtained UFG ferrite–cementite steels without martensite phase showed high strain rate sensitivity in flow stress. The UFG ferritic steels are expected to have high potential to absorb crash energy when applied to automobile body.     

Ageing behavior of a Cu-bearing ultrahigh strength steel

On ageing at different temperatures a various combination of properties has been obtained for this Cu-bearing ultrahigh strength steel. A substantial increase in strength has been obtained at 450 °C, accompanied by a drop in percentage elongation, percentage reduction in area and toughness. At 550 °C temperature extensive -Cu precipitates have been observed. The increased strength value retained in the temperature range of 450–600 °C and a secondary hardening peak obtained at 600 °C is probably due to the formation of fine Mo carbide precipitates. The decrease in strength at 650 °C along with an increase in percentage elongation, percentage reduction in area and toughness is due to the coarsening of Cu particles and a partial recovery of matrix. At 700 °C most of the Cu precipitates become rod shaped and formation of fresh martensite with a dark contrast is observed at the lath boundaries.     

Influence of deep cryogenic treatment on the thermal decomposition of Fe–C martensite

The beneficial effects of deep cryogenic treatment (DCT) at temperatures close to ?180 °C on certain mechanical properties of steels are well known, although the metallurgical base mechanism of DCT still needs further clarification. In this study, the thermal decomposition of steel martensite (100Cr6) subjected to low-temperature soaking over different periods (SDCT = 5 min at ?180 °C, LDCT = 24 h at ?180 °C) is investigated by means of differential scanning calorimetry and dilatometry. The results were compared with those for the same conventionally quenched and tempered steel. Isochronal annealing experiments at different heating rates were performed, in order to highlight the main tempering stages and to obtain their relevant activation energies. DCT was clearly shown to lower the Ea of the pre-precipitation process more intensely than in the quenched steel. This result may probably be ascribed to an increased dislocation density and to the activation of the carbon segregation process in larger amounts of martensite. The precipitation of transition carbides was also enhanced by the low-temperature conditioning of martensite. As expected, DCT transformed the retained austenite, so that the corresponding peaks almost disappeared from both the dilatometric and the DSC patterns.     

Cu对9Ni钢强度和低温韧性的影响

研究了Cu含量(质量分数)对9Ni钢强度和低温韧性的影响,并结合显微组织观察和精细结构分析了含铜9Ni钢的强韧化机理.结果表明,经过淬火+两相区淬火+回火(QLT)处理,Cu含量由0提高到1.5%,9Ni钢的室温屈服强度和抗拉强度分别提高约150和105 MPa;随着Cu含量的提高-196℃低温冲击功呈现先增加后降低的趋势,Cu含量为1.0%时达到最高值157 J,而所有含铜9Ni钢的冲击功均保持在较高的水平。随着Cu含量的增加,钢中二次回火马氏体增加而铁素体减少;颗粒或短杆棒状Cu析出物在基体上析出,组织强化与析出强化共同使钢的强度提高。同时,Cu的加入提高了二次回火马氏体板条边界上的逆转奥氏体含量,并富集于逆转奥氏体中提高其稳定性,从而提高了钢的低温韧性。     

Effect of Si on mechanical properties of low alloy steels

Abstract

The effect of Si content on mechanical properties in 0·6C–(1·0–2·5)Si–2Ni–0·2V (wt-%) steels was investigated using tensile tests, Charpy impact tests, and microstructural examination with transmission electron microscopy. The results showed that the tempering temperatures both for the maximum yield strength and for the softening of low alloy steel shifted to higher temperatures owing to the retardation of the conversion of ? carbide to cementite within martensite laths caused by Si addition. Additionally, it was found that increasing Si content shifted the tempered martensite embrittlement temperatures upwards, owing to the retardation of the formation and growth of cementite boundaries caused by the added Si.     

组织对含铜钢中析出行为的影响

为研究含铜钢在连续冷却过程中的析出行为,采用不同冷却速度及中间淬火温度获得不同的金相组织.利用金相显微镜和透射电镜研究不同组织对析出的影响.实验结果表明：当冷却后组织为铁素体时,随着铁素体的产生,在铁素体内产生第二相析出颗粒,当冷却产物为贝氏体及马氏体时,并未观察到第二相析出.当相变后产物为多种组织时,仅在铁素体中发现...     

Effects of alloying elements on microstructural evolution and mechanical properties of induction quenched-and-tempered steels

The effects of Cr and/or Mo additions and tempering temperatures on mechanical properties in relation to the microstructural evolution during tempering were investigated in induction-tempered steels. The additions of Cr and/or Mo result in the finer distribution of cementite particles due to the decrease in the coarsening rates of cementite particles above tempering temperature of 400°C, while their influence is less effective at low tempering temperatures. Accordingly, the increments of tensile strength and yield strength by the addition of alloying elements become more pronounced at high temperatures above 400°C. The occurrence of maximum peak of yield strength at 400°C would be related to further precipitation of the cementite at low temperatures, and the subsequent spheroidization and coarsening process of the cementite at high temperatures. The addition of alloying elements does not change the minima in Charpy impact values, related to tempered martensite embrittlement, since alloying elements do not have an influence on the decomposition of retained austenite and the formation of the cementite at boundaries. The strain-hardening exponent, n, decreases up to 400°C and then continuously increases with tempering temperature. This abrupt increase of n at 300°C is related to the transformation of retained austenite during straining in induction-tempered steels.     

Microstructural characterization of shielded metal arc weldments of a copper-bearing HSLA stee

Abstract

A systematic microstructural characterization in the heat-affected zone (HAZ) of two ASTM A710 grade A steel weldments (one preheated and the other pre–cooled), employing identical shielded metal arc welding conditions, has been performed. The microstructure in both the HAZ and the weld metal of both welds has been characterized by optical, scanning, and transmission electron microscopy in conjunction with microhardness traverses. No difference in microstructure was observed in the HAZ on comparing the preheated and non-preheated weldments. The only significant difference observed in the two weldments was the width of the HAZ, which is about 1 mm wider for a preheated weldment. Examination by transmission electron microscopy revealed the following microconstituents in the HAZ of both the weldments: polygonal ferrite, acicular ferrite, ferrite–carbide aggregates, ε-copper and fine cementite precipitates, martensite, tempered martensite, retained austenite, and transformation-twinned martensite. The microhardness traverse revealed almost identical hardness gradients in the two welds. The microstructural and microhardness data are discussed with regard to the preheating requirements for this alloy.

MST/118     

Influence of torsion deformation on microstructure of cold-drawn pearlitic steel wire

A comparative microstructural analyse of cold-drawn pearlitic steel wires in as-drawn and after an additional torsion deformation states is presented in this paper. During torsion the temperature of the wire increases to attain 90 °C. Then the microstructure of wires is the result of different events effects, as initial drawing, temperature increase and torsion deformation. Individually or in association, both events influence the stress level and nature in ferrite and cementite lamellae, modify the kinetic of cementite decomposition and change the dislocation mobility in cementite and ferrite. Carbon atoms migration from cementite to ferrite is affected by these thermomechanical treatments inducing a modification of dislocation pinning by carbon atoms and lamellae interfaces. The phases’ determination and quantification, associated with the carbon content variation in each phase was investigated by Mössbauer spectroscopy. The evolution of the pearlitic steel wires microstructure will be discussed point-by-point, as a function of applied deformation nature.     

The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

The effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.     

Influence of cementite spheroidization on relieving the micro-galvanic effect of ferrite-pearlite steel in acidic chloride environment

The corrosion behavior of the as-received steel and the spheroidized steel in acidic chloride environment was investigated. The results indicate the corrosion mode and corrosion rate of two steels are diverse due to their difference in microstructure. For as-received steel with ferrite-pearlite microstructure, severe localized corrosion happens on the pearlite regions, and plenty of cathodic cementite remains in the pits, further strengthening the micro-galvanic effect and accelerating the corrosion rate. While for spheroidized steel with tempered martensite microstructure, the nanosized cementite particles evenly distributed on the ferrite substrate are easy to fall off, which can significantly reduce the cementite accumulation on the steel surface, relieving the acceleration effect of micro-galvanic corrosion.     

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.     

Effect of tempering temperature on the microstructure and properties of ultrahigh-strength stainless steel

The microstructure, precipitation and mechanical properties of Ferrium S53 steel, a secondary hardening ultrahigh-strength stainless steel with 10% Cr developed by QuesTek Innovations LLC, upon tempering were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile and impact tests. Based on these results, the influence of the tempering temperature on the microstructure and properties was discussed. The results show that decomposition occurred when the retained austenite was tempered above 440 °C and that the hardening peak at 482 °C was caused by the joint strengthening of the precipitates and martensite transformation. Due to the high Cr content, the trigonal M₇C₃ carbide precipitated when the steel was tempered at 400 °C, and M₇C₃ and M₂C (5–10 nm in size) coexisted when it was tempered at 482 °C. When the steel was tempered at 630 °C, M₂C and M₂₃C₆ carbides precipitated, and the sizes were greater than 50 nm and 500 nm, respectively, but no M₇C₃ carbide formed. When the tempering temperature was above 540 °C, austenitization and large-size precipitates were the main factors affecting the strength and toughness.     

Suppression of hydrogen-induced blisters in SK4 carbon steel alloy by friction stir processing

The feasibility of using friction stir-processing technique as a microstructural refining method for preventing hydrogen-induced blisters of SK4 carbon steel alloy containing microstructure of hard cementite particles in a ductile ferrite matrix was examined. Amount and size of hydrogen-induced blisters decreased in the stirred layers with increasing rotation speed and completely disappeared when rotation speed attained 400 rpm because of the formation of a hard and fine microstructure consisting of pearlite, martensite, and retained austenite instead of that containing hard spheroidized cementite particles in a soft ferrite matrix. Interfaces between ferrite matrix and cementite particles were the preferential sites for the hydrogen blisters initiation.