Martensite-austenite Constituent Research Articles

Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated heat-affected zones of Mn-Mo-Ni low-alloy steels

This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M2C carbides was greatly increased and the number of coarse M3C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M2C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.

Mechanical and metallurgical investigation of martensite–austenite constituents in simulated welding conditions

It is well known that the martensite-residual austenite or “M–A” constituent in the heat affected zone (HAZ) is a major factor decreasing the toughness of the welded joint.Up to now, many researches have been devoted to the effect of metallurgical factors and the influence of fraction of M–A on the toughness value.This study entails the influence of thermal cycles and also the chemical composition of the structural steels on the formation and decomposition of M–A constituents by multipass welding conditions simulated on a dilatometer and/or a Gleeble simulator. Additionally, certain experimental results will be presented and the comparative evaluations will be discussed.

An ultra low carbon Cu bearing steel: influence of thermomechanical processing and aging heat treatment on structure and properties

An ultra low carbon Cu bearing steel, micro alloyed with Nb was thermo-mechanically processed. Variation in microstructures and associated mechanical properties at different finish rolling temperatures was studied. Microstructures in hot rolled steels showed a mixture of acicular ferrite and bainitic ferrite with inter-lath or intra-lath precipitation of microalloying NbC/Nb(CN) precipitates. Second phase particles were observed, mostly at lath boundaries and were identified as retained austenite, twinned martensite islands or martensite austenite constituents. Variation in strength at different finish rolling temperatures was due to variation in volume fraction of bainitic ferrite and dislocation substructure in the matrix. Precipitation behaviour of Cu was studied by differential scanning calorimetry and transmission electron microscopy. At peak age hardening condition, coherent bcc Cu particles were formed and activation energy for this reaction was evaluated. At higher aging temperature, growth of fcc e-Cu particles and recovery of structure occurred which decreased the strength value.

Mechanisms and modeling of cleavage fracture in simulated heat-affected zone microstructures of a high-strength low alloy steel

The effect of the welding cycle on the fracture toughness properties of high-strength low alloy (HSLA) steels is examined by means of thermal simulation of heat-affected zone (HAZ) microstructures. Tensile tests on notched bars and fracture toughness tests at various temperatures are performed together with fracture surface observations and cross-sectional analyses. The influence of martensite-austenite (M-A) constituents and of “crystallographic” bainite packets on cleavage fracture micromechanisms is, thus, evidenced as a function of temperature. Three weakest-link probabilistic models (the “Master-curve” (MC) approach, the Beremin model, and a “double-barrier” (DB) model) are applied to account for the ductile-to-brittle transition (DBT) fracture toughness curve. Some analogy, but also differences, are found between the MC approach and the Beremin model. The DB model, having nonfitted, physically based scatter parameters, is applied to the martensite-containing HAZ microstructures and gives promising results.

Effect of cooling rate on the as-quenched microstructure and mechanical properties of HSLA-100 steel plates

The effect of cooling rate on the as-quenched microstructure and mechanical properties of a 14-mm-thick HSLA-100 steel using various cooling media such as brine, water, oil, air, and furnace has been studied. While quenching in brine, water, and oil resulted in lath martensite structures, the granular bainite and martensite-austenite (M-A) constituents were found in air- or furnace-cooled specimens. The average lath spacing increased slightly on decreasing the cooling rate (300 nm in brine-quenched specimen to 400 nm in oil-quenched specimen). The precipitates of Cu and Nb(C, N) were observed in all the quenching conditions except in the brine-quenched specimen. The as-quenched strength and toughness of the brine-, water-, and oil-quenched specimens were higher (yield strength: 894 to 997 MPa, ultimate tensile strength: 1119 to 1153 MPa, and Charpy V-notch energies: 65 to 70 J at −85 °C) than those of air- and furnace-cooled specimens (yield strength: 640 to 670 MPa, ultimate tensile strength: 944 to 1001 MPa, and Charpy V-notch energies: 10 to 20 J at −85 °C). For industrial production of HSLA-100 steel plates, oil or water quenching is recommended in lower thickness plates (<25 mm). For production of thicker plates, however, water quenching is more suitable.

Evolution of microstructure in an ultra‐low carbon Cu bearing HSLA forging

Over the last decade low carbon age hardenable steels have become a subject of interest for naval applications. In the present study an ultra‐low carbon Ni‐bearing steel alloyed with Cu was cast and processed by controlled forging and subsequently cooled in air, sand and water. A variety of multiphase structures have been observed because of different cooling rates. The slowest cooling rate has produced bainitic ferrite by ledge mechanism whereas lath‐ or plate‐type bainitic ferrite was predominant at the faster cooling rates. The volume fraction of retained austenite and/or martensite austenite constituents increased with decreasing cooling rate. Precipitation of finer micro‐alloying carbides and carbonitrides was suppressed by faster cooling. Slow cooling rates also resulted in the formation of ɛ‐Cu precipitates of varying sizes and shapes.

Influence of martensite-austenite constituents on heat affected zone toughness of C-Mn steel welds

"Martensite-retained austenite" constituents play an important role on Heat Affected Zone (HAZ) toughness in the welding of structural steels. The influence of thermal cycles - especially in the case of multipass welding - and of steel composition on the presence of such constituents is discussed here. Fracture initiation mechanisms related to these constituents are also examined.

Microstructure and toughness of intercritically reheated heat affected zone in reactor pressure vessel steel weld

This study is concerned with correlating the metallurgical factors and the toughness of the intercritically reheated coarse grained heat affected zone (ICCGHAZ) of multipass welded SA508–cl. 3 reactor pressure vessel steel. The influence on toughness was investigated by means of a simulated heat affected zone test. Toughness and other properties of the ICCGHAZ were strongly influenced by metallurgical factors such as high concentration of carbon martensitic islands (secondary phase, martensite–austenite constituents) formed along the prior austenite grain boundaries and martensite lath interfaces, and a softened matrix (tempered martensite or bainite). The characteristics (amount, hardness, and size) of the islands were found to be strongly correlated with both peak temperature and the cooling time of the previous pass. The toughness of the ICCGHAZ deteriorated with increasing amounts of martensite–austenite constituents, which were made more abundant by increasing the final peak temperature within the intercritical temperature range. Meanwhile, for the same intercritical peak temperature, toughness decreased with increased cooling time. When the cooling time was short, the dominant factor influencing toughness of the ICCGHAZ was the amount of martensitic islands. However, when the cooling time was long, the hardness difference between the martensitic islands and the softened matrix (tempered martensite) was found to be the dominant factor.

Microstructure of martensite–austenite constituents in heat affected zones of high strength low alloy steel welds in relation to toughness properties

Microstructure and fracture properties relationships have been investigated in the heat affected zones (HAZs) of a high strength low alloy steel used for offshore applications. Metallographic examinations of simulated HAZ microstructures were conducted to investigate the detailed microstructure of the martensite–austenite (M–A) constituents. Using scanning and transmission electron microscopy, various microstructures were found for the M–A constituents. In mixed particles, retained austenite was located at the periphery of the islands. Chemical and/or mechanical effects could possibly account for the stabilisation of this austenite phase. In situ cooling experiments in the transmission electron microscope showed that stacking faults play an important role in the thermal stability of austenite. Impact properties of simulated HAZ microstructures are strongly affected by both the bainitic microstructure and M–A constituents. In particular, freshly transformed high carbon martensite was shown to be much more deleterious than retained austenite.

Effect of Nitrogen on Formation of Martensite-Austenite Constituent in Low Carbon Steels.

The effect of dissolved nitrogen on phase transformations and microstructural evolution in low-carbon high manganese silicon steel wire-rod was examined for varying nitrogen levels of 30, 70, and 100 ppm, while keeping the base composition constant. It was found that nitrogen suppresses the pearlite reaction start temperature by up to 30°C. A strong correlation was discovered between the nitrogen level and the formation of martensite-austenite microconstituent (MA phase). The possible mechanisms by which nitrogen causes the formation of MA phase are discussed. The main conclusion drawn from this study is that levels of nitrogen between 70 and 110 ppm can have a significant effect on microstructure, such that up to approximately 10vol% of MA phase is formed in these steels. Microstructural changes in titanium-microalloyed steels were also investigated - particularly changes in the development of the MA and pearlite phases. The pearlite content was up to double that found in a Ti-free alloy. Microalloying with titanium was found to be effective in completely eliminating MA phase.

構造用鋼 ベイナイト型非調質鋼の組織と機械的性質に及ぼす合金元素の影響と切欠き材の疲労強度

Influence of carbon, manganese, chromium, and vanadium contents on mechanical properties and microstructures of bainitic microalloyed steels are investigated. The fatigue limit of the bainitic steels were compared with the ferritic-pearlitic steels with smooth and notched specimens. The hardness are increased by increase in content of each alloying element. The carbon equivalent for hardness (Ceq) can be described by the following equation Ceq=C(mass%)+0.23 ×Mn(mass%)+0.26 ×Cr(mass%)+0.31 × V(mass%). Carbon, manganese, and chromium decrease toughness but vanadium improves both toughness and hardness. Additionally, vanadium is effective in improvement of 0.2% proof. This reason is estimated that microstructures of bainitic steel i.e. martensite-austenite constituent (M-A constituent) are refined by vanadium addition. The fatigue strength of bainitic steels with smooth specimen is lower than ferritic-pearlitic steels for the same hardness. However, it is higher in the case of notched specimen. This is because bainitic steels have better notch sensitivity.

Prevention of HAZ cracking in corner joints of high heat input SAW box columns

Summary One-pass submerged-arc welding (SAW) with high heat input is extensively used for the corner welds of box columns, though internal cracking of lamellar tearing type in some cases occurs along the centreline of the flange plate thickness. This paper describes an investigation of weld microstructures, hydrogen concentrations, and residual stresses at the cracking position to clarify the cracking mechanism. Appropriate measures to prevent cracking in terms of material- and fabrication-related factors are also studied on the basis of laboratory-scale one-pass SAW experiments and numerical analysis of hydrogen diffusion. The generation and propagation of cracking are closely related to the transition of the hydrogen concentration at the cracking position, whereas the effect of the residual stress on cracking is small. The material-related factors controlling this cracking include elongated manganese sulphide (MnS) in the centre segregation band and martensite-austenite constituent (M-A) around the MnS formed by the intercritical heat effect of one-pass SAW. Moisture contained in the flux is the greatest source of hydrogen, so that flux drying is the most effective preventive measure affecting the welding conditions. To prevent cracking, a bevel groove for a one-pass corner joint SAW is preferable to a V groove. PWHT at 300°C for more than 2 hours is also effective in this respect. Effective fabrication-related preventive measures include alleviation of centre segregation by hardenability reduction in continuous casting, shape control of sulphides by the addition of Ca, and Ceq reduction through application of thermomechanical control.

大入熱ＳＡＷ Ｂｏｘ柱のＨＡＺ割れ防止対策の検討

One-pass submerged arc welding (SAW) with high heat-input is widely employed for the corner seam of box-shaped columns, however, an internal crack similar to lamellar tearing occasionally occurs along the center-thickness line of the flange plate. In this paper, microstructures, hydrogen concentrations and residual stresses at the cracking position have been investigated to clarify the mechanism of the cracking. In addition, appropriate countermeasures against the cracking in terms of the quality of plates and the conditions of welding have been studied, by application of the one-pass SAW experiment in the laboratory and the numerical analysis of hydrogen diffusion.The generation and the propagation of the cracking were closely related to the transit of hydrogen concentration at the cracking position, while the effect of residual stresses on the cracking was small. The dominant material factors of this cracking were elongated manganese sulfides (MnS) in the center segregation band, and martensite-austenite constituents (M-A) around MnS formed by intercritical heat affection of one-pass SAW.Moisture contained in the flux was the greatest source of hydrogen, so that drying of the flux was the most effective countermeasure in the welding conditions. As a groove for the one-pass SAW corner joint, a bevel groove was preferable to a V groove to prevent cracking. PWHT at 300°C for longer than 2 hours was also effective. As countermeasures in the production of steel plates, alleviating center segregation by soft reduction in continuous casting, shape control of sulfides by addition of Ca, and lowering Ceq by application of thermo-mechanical-control-process were effective.

Microstructures relevant to brittle fracture initiation at the heat-affected zone of weldment of a low carbon steel

Charpy toughness of the heat-affected zone (HAZ) of weldment of a low carbon steel has been investigated by means of an instrumented Charpy test and fractographic analysis. Microstructures were varied with thermal cycles simulating double-pass welding. The ductile-brittle transition temperature is the most deteriorated at an intermediate second-cycle heating temperature. The origin of the difference in the transition temperatures has been analyzed to exist in the brittle fracture initiation stage. Fractographic examination correlating with microstructural features has revealed that the brittle fracture initiation site is associated with the intersection of bainitic ferrite areas with different orientations rather than the martensite-austenite constituents. The role of the constraint of plastic deformation on the brittle fracture initiation is discussed.

A metallographic technique for detecting martensite-austenite constituents in the weld heat-affected zone of a micro-alloyed steel

The industrial demand for weldable high-strength low-alloy steel with good toughness in the weld heat-affected zone, even when high values of heat input are employed, has significantly increased in the last years. Several publications have indicated that the martensite-austenite island is the main microstructural constituent controlling this toughness, highlighting the need for a precise identification of these constituents in studies dealing with the microstructure-toughness relation. In this paper, a new metallographic technique is presented that enables a quick and accurate identification of this constituent by both light optical and scanning electron microscopy. The study was conducted in the weld simulated heat-affected zone of a micro-alloyed Nb steel with small additions of Cu and Ni; a C-Mn steel with low carbon was employed as a reference.

８０及び１００キロ級高張力鋼大入熱ＨＡＺ部の靭性劣化とその改善に関する研究（第２報） 島状マルテンサイトの金属学的特性に関する検討

The internal microstructure, chemical composition, and hardness of M-A (Martensite-Austenite constituent) formed in weld CGHAZ (Coarse Grained Heat Affected Zone) of 780-980 MPa class HSLA steels have been investigated in order to reveal metallographic characteristics of the M-A constituent. The martensite was classified into lath and twin type. The massive M-A constituent had higher C contents, and included more twin type martensite and retained austenite than the elongated M-A. The cementite was found to be classified into a coarse rod type and a dendritic or fine needle type. The coarse rod type is considered to be precipitated directly from the austenite. The dendritic or fine needle type is probably precipitated by the self tempering of martensite.The C content of the M-A constituent was increased with Δt8/5 (cooling time from 1073 to 773 K) and reached 1.3-2.2% for Δt8/5=100-1000s.The hardness of the M-A constituent was increased with C content. The hardness of the massive M-A constituent (Hv-950) was generally higher than that of the elongated M-A constituent (Hv-700). The hardness and C content of the M-A constituent can be reduced significantly by a post weld heat treatment at temperatures from 623 to 773 K.

Microstructure and Mechanical Properties of Bainite Containing Martensite and Retained Austenite in Low Carbon HSLA Steels

The microstructure of a new low carbon HSLA steel with a high tensile strength and a high toughness in the as-hot forged condition for automotive parts was observed in detail in an 0.12% C-2% Mn-1% Cr-0.25% Mo-0.12% Nb-0.021% Ti-0.0018% B steel. Lath-like microstructure observed at the cooling rate ranging from 250 to 6 o C/s mainly was revealed to consist of bainitic-ferrite with retained austenite and auto-tempered martensite. Granular microstructure observed at the cooling rate ranging from 0.17 to 0.04 o C/s was characterized tobe martensite-austenite constituent islands in featureless matrix

低炭素低合金鋼の溶接熱影響部の限界ＣＴＯＤ値に関する研究

Crack Tip Opening Displacement (CTOD) tests were carried out to evaluate the Heat Affected Zone (HAZ) toughness. The present study was focused on the main factors to affect the critical CTOD values in the HAZ from viewpoint of microstructures.It was observed that the fracture initiation properties of HAZ were related to the properties of the Local Brittle Zone (LBZ) near fusion line. The relationship between them was investigated from the viewpoint of austenite grain size, fracture facet size and fraction of Martensite-Austenite (M-A) constituent in the LBZ.The results obtained were as follows:(1) The crack initiation properties were strongly affected by the microstructures of the HAZ.(2) The LBZ deteriorated the critical CTOD values in the HAZ because of both the coarsened grain size and existing M-A constituent.(3) The fine ferritic microstructures improve the critical CTOD values in the HAZ, through reducing the fraction of M-A constituent.(4) A good correlation was found between the critical CTOD values in the HAZ and fracture facet size, although the effect of fracture facet size on the critical CTOD values in the HAZ was different for the fraction of M-A constituent.(5) The close correlation was found between the critical CTOD values in the HAZ and the fraction of M-A constituent.(6) Griffith crack was observed at M-A constituent and propagated along M-A constituent/matrix interfaces, resulting in low critical CTOD values.

高張力鋼溶接部の破壊発生・伝ぱに及ぼす島状マルテンサイトの影響

Fracture behavior of the weld zone of 80kg/mm2 class HSLA steel has been investigated by means of tensile test, Charpy impact test and three point bending test. The results obtained gave convincible evidences that showed the following important role played by the martensite-austenite (M-A) constituent on the initiation and propagation processes of cracks.For ductile fracture, cracks were initiated by cracking flaky M-A constituent or debonding blocky M-A constituent from ferrite matrix. Then cracks expanded to voids and developed to deep holes. The main crack was formed by coalescence of deep holes by internal necking and led to fracture of specimen with a ductile deep dimple fracture surface.On the other hand, for brittle fracture, cracks nucleated at the interface of blocky M-A constituent and ferrite matrix. Then the crack propagated into the specimen, resulting in catastrophic fracture with a brittle river pattern surface.

Development of grade API X80 pipeline steel plates produced by controlled rolling

The future demand for high-grade pipeline materials (API X80 and higher) calls for the development of steels with microstructures showing acicular phases (acicular ferrite, bainite, and martensite-austenite (M A) constituent). A systematic investigation has heen made (of the mechanical properties and microstructures exhibited hy a wide range of controlled-rolled plates (of final thickness 18 mm) of various chemical compositions, produced on a laboratory scale. Low-carhon (∼ 0·06% C) boron-free and boron-containing steels have been examined, as has a boron steel with a Imrer carbon content (0–02% C). The boron-free steels (carbon equivalent ≤0.46%) always exhihited a polygonal ferrite matrix with a dispersed phase consisting mainly of M A constituent, while the horoncontaining steels (carhon equivalent ≤0.40%) invariahly showed an acicular matrix with a lower volume fraction of finely dispersed M A island. Quantitative relationships are ohtained hetween the volume fractions of the microstructural components and the mechanical properties. These relationships clearly show that the type of microstructure found in the horon steels gives better strength toughness combinations (in terms of the Charpy-V shelf energy). With both the boron-free and the boron-containing steels, the best results have been obtained after adding ∼0·10% Ti (together with ∼0·03% Nb), rather than vanadium, to increase precipitation strengthening. This increase has been confirmed directly for polygonal ferrite hy quantitatively comparing the number and size distrihution of precipitates in a Nb-V and a Nb-Ti steel having otherwise identical compositions.