Original Austenite Grain Research Articles

Effect of cooling time t8/5 on microstructure and toughness of Nb–Ti–Mo microalloyed C–Mn steel

In order to further optimize welding process of Nb–Ti–Mo microalloyed steel, welding thermal cycles on coarse-grained heat-affected zone (CGHAZ) of welded joints were simulated using Gleeble 1500. The microstructure and low-temperature impact fracture were investigated using a scanning electron microscope and a pendulum impact machine, respectively. Moreover, the relationship between cooling time t8/5 and the microstructure of CGHAZ was discussed, and the effect of microstructure on impact toughness was also studied. As cooling time increased, martensite fraction decreased from 97.8% (3 s) to 3.0% (60 s). The fraction of martensite/austenite (M/A) constituent increased from 2.2% (3 s) to 39.0% (60 s), its shape changed from granular to strip, and the maximum length increased from 2.4 μm (3 s) to 7.0 μm (60 s). As cooling time increased, the prior austenite grain size increased from 34.0 μm (3 s) to 49.0 μm (60 s), the impact absorption energy reduced from 101.8 J (5 s) to 7.2 J (60 s), and the fracture mechanism changed from quasi-cleavage fracture to cleavage fracture. The decreased toughness of CGHAZ was due to the reduction of lath martensite-content, coarsening of original austenite grain, and increase and coarsening of M/A constituent. The heat input was controlled under 7 kJ cm−1 during actual welding for these steels.

Microstructure, hardness and contact fatigue properties of X30N high nitrogen stainless bearing steel

Microstructure, hardness and fatigue properties of X30N high nitrogen stainless bearing steel were investigated. It was found that nitrogen addition could effectively reduce the amount and size of coarse carbides. The original austenite grain size was obviously refined. Additionally, more retained austenite was found in X30N steel after quenching at 1050 °C, which could be reduced from about 30% to about 6% by cold treatment at − 73 °C and subsequent tempering, and thus, the ultimate hardness was increased up to about 61 HRC with reduction of austenite and precipitation of carbonitrides. Furthermore, the rolling contact fatigue lives of X30N steel are superior to those of 440C steel, which was attributed to the enhanced hardness and a certain retained austenite in the high nitrogen steel.

Effect of laser scanning speed on microstructure and wear properties of T15M cladding coating fabricated by laser cladding technology

In this paper, a layer of T15M high speed steel alloy powder was deposited on the Q235 substrate surface by laser cladding technology to ensure appropriate scanning speed range serving the production of paper coating scraper. The microstructure of different scanning speeds (100 mm/min, 200 mm/min, 300 mm/min) laser cladding layers was studied by XRD, SEM, EDS and EPMA e.g. the wear properties of the coatings were studied by sliding wear test. The results show that T15M composite coating comprised martensite, retained austenite and carbides. With the increase of the laser scanning speed, more carbides precipitated and large local segregation decreased in the original austenite grain boundary. Defects like pore, microcrack and inclusion existed in the coating. The coating with a scanning speed of 200 mm/min had tiny grain size with high solid solubility compared with the other two coatings. Microhardness of the T15M cladding coating shows the coating with a scanning speed of 200 mm/min possessed higher average hardness in the cladding area while the coating with a scanning speed of 100 mm/min owned the lowest hardness. Sliding wear test indicates that the wear performance of the coating deteriorated and the debris was mass flaking when the laser scanning speed reached 300 mm/min. However, when scanning speed was 200 mm/min, the best wear resistance with obvious plastically deformed furrows occurred.

Microstructural Factors That Decrease the Local Strength of Grain Boundaries in Martensitic Steels

As a result of the phase transformation of austenite to martensite during steel quenching, weakened structural regions, specifically the boundaries of the original austenite grains, have been formed. They are weakened because of microstructural factors, such as the residual internal microstrains and segregation of embrittling impurities. The joint effect of microstructural factors, namely, residual microstrains and segregation of phosphorus and carbon at grain boundaries, on reducing the local strength of the boundaries of the initial austenite grains in martensitic steels is quantitatively evaluated, and the impacts of these microstructural factors have been separated. The dependences of the local grain-boundary strength on the ratio of various levels of residual microstrains and on the atomic concentration of phosphorus impurities at the grain boundary in segregation spots have been determined. It has been shown quantitatively that the adsorption enrichment of the austenite grain boundaries with phosphorus leads to a decrease in the intergrain adhesion and facilitates the emergence and development of cracks along the boundaries of the initial austenite grains. The quantitative dependence of the local strength of grain boundaries on the concentration ratio of carbon and phosphorus in them has been shown. Carbon in concentrations of up to 0.04% reduces the embrittlement of the boundaries due to the segregation of phosphorus and loses its neutralizing effect on the phosphorus segregation at concentrations of more than 0.04%, so the phosphorus concentration at the grain boundaries increases and the embrittlement resistance of the latter decreases. The applicability of the developed technique for the quantitative evaluation of the local strength of hardened steel grain boundaries by using tests on delayed fracture and applying the method of finite elements to determine the local strains has been shown.

Microstructures and their distribution within HAZ of X80 pipeline steel welded using hybrid laser-MIG welding

Large microstructure gradient in the heat-affected zone (HAZ) in the joints of large thick material welded by the hybrid laser-MIG welding technology reduces the in-service reliability and durability of the welded structure. It is of great importance to characterize and analyze how the microstructure distribution and evolution occur in the HAZ of the laser-MIG hybrid welded joints fabricated with X80 pipeline steel. In this article, the HAZ is found to comprise these characteristic zones, namely banded microstructure HAZ (BMHAZ), fine-grained HAZ (FGHAZ), transitional microstructure HAZ (TMHAZ), and coarse-grained HAZ (CGHAZ). The zone of the HAZ contains quasi-polygonal ferrite (QF), M-A component, polygonal ferrite (PF), and bainite ferrite (BF). From the base metal side towards the weld center with the decrease in the distance, the size of the M-A components decreases and its distribution is more dispersed, while the content of QF decreases in the HAZ. The average diameter of the original austenite grain increases gradually and so does the content of lath microstructures.

Influence of original austenite grain size on tensile properties of a high-manganese transformation-induced plasticity (TRIP) steel

Original austenite grain size and stacking fault energy (SFE) of a high-Mn transformation-induced plasticity (TRIP) steel were varied by annealing at different temperatures. The microstructural evolution during cooling and tensile deformation were investigated to explain the work hardening behavior. The study suggested that the amount of ε-martensite was increased with annealing temperature. When SFE was greater than 13.3mJ/m2, twin may form in austenite on cooling; and ε-martensite cannot be thermally induced in austenite once its SFE was higher than 20.6mJ/m2. Work hardening behavior of the steel annealed at different temperatures could be divided into two stages using Hollomon analysis. The grain refinement strengthening and deformation induced γ → ε transformation rendered 800°C annealed sample to have the largest work hardening exponent in Stage-I, in this case ultimate tensile strength was also highest. In Stage-II, significant deformation-induced ε → α′ transformation in samples annealed at high temperatures resulted in greater improvement of work hardening exponent. The elongation of the steel was also improved with annealing temperature because local stress concentration was effectively released by ε → α′ transformation.

Overall Hardening of Solid-Rolled Wagon Wheels by Volume Quenching and Surface Plasma Processing

In relation to solid-rolled wagon wheels the advantages of differentiated quenching of all wheel elements are shown in comparison with quenching of only wheel rim rolling surface. It should be noted that larger possibilities to enhance performance characteristics of heavy duty details and products can be fulfilled using the technology of overall hardening including the differentiated quenching and surface plasma processing. It is established that along with the improvement of mechanical properties plasma processing after volume quenching allows for a considerable increase of crack resistance of surveyed grades in comparison with volume quenching without plasma processing. The martensite structure of the strengthened layer during such overall processing in comparison with the martensite structure of volume and strengthened base is characterized by high level of particle size that is specified by reduction of the original austenite grain due to very high heating and cooling speeds as well as small time of steel stay at high temperatures. In order to increase hardness and resistance against cracks generation plasma processing and preliminary volume quenching are recommended. A composite working layer with high wear and crack growth resistance and relatively soft and plastic core are developed in details and products volume quenched when surface plasma strengthening.

Effect of Ti on T15M composite coating fabricated by laser cladding technology

In this article, the T15M (belonging to a kind of high speed steel powder) composite coating with different Ti content was prepared by laser cladding technology (LCT). The microstructure and properties of the samples were investigated by scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), automatic micro hardness tester and multifunctional friction and wear testing machine. The results show that the microstructure of the T15M composite coating is mainly composed of martensite, austenite and carbides which mainly distribute on the original austenite grain boundaries. With the increase of titanium element, the shape of carbides become more spherical and the carbides on the grain boundaries change from continuous to discontinuous which attributes to the destruction of carbon concentration balance at the interface. In contrast to the T15M composite coating without adding titanium, the carbides distribution in the coating are more uniform relying on preferential homogeneous nucleation of titanium compounds in solidification. By hardness test, it can be discovered that the addition of titanium to T15M composite coating has no significant effect on the average hardness, but the fluctuation range of the microhardness value decreases. After the sliding wear test, both the coefficient of friction and the mass loss after wear are reduced with the addition of titanium.

Comparison of microstructure and property of high chromium bearing steel with and without nitrogen addition

Microstructure and property of bearing steel with and without nitrogen addition were investigated by microstructural observation and hardness measurement after different heat treatment processing. Based on the microstructural observation of both 9Cr18 steel and X90N steel, it was found that nitrogen addition could effectively reduce the amount and size of coarse carbides and also refine the original austenite grain size. Due to addition of nitrogen, more austenite phase was found in X90N steel than in 9Cr18 steel. The retained austenite of X90N steel after quenching at 1050 °C could be reduced from about 60% to about 7% by cold treatment at −73 °C and subsequent tempering, and thus finally increased the hardness up to 60 HRC after low temperature tempering and to 63 HRC after high temperature tempering. Furthermore, both the wear and corrosion resistance of X90N steel were found much more superior than those of 9Cr18 steel, which was attributed to the addition of nitrogen. It was proposed at last that nitrogen alloying into the high chromium bearing steel was a promising way not only to refine the size of both carbides and austenite, but also to achieve high hardness, high wear property and improved corrosion resistance of the stainless bearing steel.

Identification and Characterization of Intercritical Heat-Affected Zone in As-Welded Grade 91 Weldment

A metallurgical method is proposed for locating the intercritical heat-affected zone in the as-welded Grade 91 steel. New austenitic grains, preferentially formed along the original prior austenite grain boundaries, are characterized to contain finer M23C6 carbides and higher strain levels than the original prior austenite grains. Kurdjumov–Sachs Group 1 variant pairs, with a low misorientation of 7 deg within a martensitic block, are identified as the dominant variants in the new PAGs.

Crystal structure of modulated martensite and crystallographic correlations between martensite variants of Ni50Mn38Sn12alloy

A comprehensive study on the crystal structure, the microstructure and the crystallographic features of the martensite in an Ni50Mn38Sn12alloy has been conducted in the present work. The results show that the martensite possesses a 4O modulated structure. The martensite is organized into broad plates in the original austenite grain. The plates contain irregularly shaped colonies with two characteristic microstructural patterns: a classical lamellar pattern and a herringbone pattern. Crystallographic analyses by scanning electron microscopy/electron backscatter diffraction demonstrate that in each colony there are four orientation variants (A, B, C and D) and they form three types of twins (type I, type II and compound twin). The interfaces between corresponding variants are coincident with their twinning planeK1. The interface planes of the compound twin pairs A&D and B&C can have one or two different orientations, which leads to the two microstructural patterns. The corresponding variants in neighboring colonies within one broad plate (intra-plate colonies) possess close orientations, but the type I and the type II twin relationships are interchanged. The variants in neighboring colonies situated in adjacent plates (inter-plate colonies) are type I or type II twin related but with some angular deviations. The plate interface is defined by the {221} plane of the variant pair with largest thickness. The results of the present work provide comprehensive microstructural and crystallographic information on modulated martensite in NiMnSn alloys that is useful for the understanding of their specific functionalities and helpful for further investigation on property optimization of these materials.

Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel

Maximum hardness test in weld heat-affected zone (HAZ), oblique Y-groove cracking test and mechanical property test of welding joint of 1000 MPa grade ultra-low carbon bainitic steel were carried out, so as to research the weldability of the steel. The results show that the steel has lower cold cracking sensitivity, and preheating temperature of 100 °C can help completely eliminate cold cracks, generating good process weldability. The increase of preheating temperature can reduce the hardening degree of heat-affected zone. The strength of welding joint decreases and hardness reduces when heat inputs increase, and excellent mechanical properties can be obtained when low welding heat inputs are used. Fine lath bainites of different orientations combined with a few granular bainites that effectively split the original coarse austenite grains are the foundation of good properties.

A comparative study of the microstructure and properties of 800 MPa microalloyed C-Mn steel welded joints by laser and gas metal arc welding

The differences in microstructure and mechanical properties of laser beam welded (LBW) and gas metal arc welded (GMAW) joints of 800MPa grade Nb-Ti-Mo microalloyed C-Mn steel of 5mm thickness were studied. The study suggested that the microstructure in welded seam (WS) of GMAW was acicular ferrite and fine grained ferrite, whereas lath martensite (LM) was obtained in WS of LBW, where inclusions were finer and did not act as nucleation sites for acicular ferrite. The microstructure of coarse-grained HAZ (CGHAZ) obtained using the two welding methods was LM and granular bainite (GB), respectively. The original austenite grain size in CGHAZ of LBW was 1/3 of GMAW. The microstructure of fine-grained HAZ and mixed-grained HAZ using the two welding methods was ferrite and M-A constituents, while that of LBW was significantly fine. The hardness of LBW welded joints was higher than the base metal (BM), which was the initiation site for tensile fracture. The tensile fracture location of GMAW welded joints was in WS. The impact toughness of LBW welded joints was excellent and the impact absorption energy was similar to BM.

Microstructure Evolution and Precipitation Behavior of 0Cr16Ni5Mo Martensitic Stainless Steel during Tempering Process

The microstructure, morphology of precipitates and retained austenite and the volume fraction of retained austenite in 0Cr16Ni5Mo stainless steel during the tempering process were analyzed using optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD) and scanning transmission electron microscope (STEM). The results show that the microstructure of the tempered steel is mainly composed of tempered martensite, retained austenite, and delta ferrite. In the case of samples tempered from 500 to 700 °C, the precipitates are mainly M23 C6, which precipitate along the lath martensite boundaries. The precipitate content increases with the tempering temperature. During the tempering process, the content of retained austenite initially increases and then decreases, the maximum content of retained austenite being 29 vol. % upon tempering at 600 °C. TEM analysis of the tested steel reveals two morphology types of retained austenite. One is thin film-like retained austenite that exists along the martensite lath boundary. The other is blocky austenite located on packet at the boundary and the original austenite grain boundary. To further understand the stability of reversed austenite, the Ni content in reversed austenite was measured using STEM. Results show a significant difference in nickel concentrations between reversed austenite and martensite.

Austenite grain refinement during load-biased thermal cycling of a Ni49.9Ti50.1 shape memory alloy

A near-equiatomic NiTi shape memory alloy was subjected to a variety of thermomechanical treatments including pure thermal cycling and load-biased thermal cycling to investigate microstructural evolution of the material under actuating conditions. In situ and post mortem scanning transmission electron microscopy (STEM) was used to study the effects of stress on the development of defect substructures during cycling through the martensitic transformation. High temperature observations of the austenite phase show rapid accumulation of dislocations and moderate deformation twinning upon thermomechanical cycling. Additionally, TEM-based orientation mapping suggests the emergence of fine crystallites from the original coarse austenite grain structure. A possible mechanism is proposed for the observed grain refinement based on the crystallographic theory of martensite transformation.

Austenite Grain Refinement by Reverse α′ → γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investigated using optical microscopy, X-ray diffraction (XRD), electrical resistivity and hardness testing. Austenite grain refinement was successfully achieved by a two-step heat treatment. First, martensite was produced by cooling the solution-treated samples to −196 °C. Then, the deep cryogenic treated samples were heated to 850 °C upon slow or rapid heating. The mean size of original austenite grain was about 400 μm. But the mean size of equiaxed reversion austenite was refined to 50 μm. Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating, and the residual austenite was decomposed, resulting in the formation of pearlite nodules at the austenite grains boundaries. The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite. However, the reverse transformation upon rapid heating was predominated by displacive manner. The residual austenite was not decomposed. The plate a-phase was carbon-supersaturated until the starting of reverse transformation. The reverse transformation was accompanied by surface effect, resulting in the formation of plate austenite with high density dislocations. The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.

Phase Transformation Behavior and Microstructural Control of High-Cr Martensitic/Ferritic Heat-resistant Steels for Power and Nuclear Plants: A Review

The martensitic/ferritic steels have been used as boiler and turbine materials in power plants, and also been selected as potential materials for structural materials in nuclear reactors. In this paper, the kinetic analysis of the martensite formation and microstructural control of high-Cr martensitic/ferritic steels are reviewed. A modular approach, incorporating Fisher partitioning nucleation and anisotropic growth for impingement, was proposed to describe the martensite formation kinetics under different cooling rates. The kinetic analysis suggested a thermal-activated growth feature occurring during the martensitic transformation of martensitic steels. The microstructure can be tuned by composition optimization and various combinations of heat treatment parameters (temperature, time, severe and minor deformation). For the application in power plant, the potential of boundary-design, refinement of original austenite grain size and the final martensitic lath, pinning effect of stable carbides, in improving the performances of martensitic/ferritic steels at elevated temperatures should be investigated more thoroughly. Furthermore, efforts should be made to explore the effects of retained austenite on the improvement of high-temperature creep strength. For the application of nuclear plants, attempts should also be made to produce Fe powders with uniformly distributed oxide particles by chemical reactions.

The Effect of Heating Process on Strength and the Original Austenite Grain Size of Hot Forming Parts

Strength, microstructure and austenitic grain size of a hot formed steel WHT1300HF after simulative hot stamping were studied by using universal testing machine for materials and optical microscopy. The results show that the yield strength of the hot stamping parts presented the tendency of earlier decrease and later increase with the extension of holding time, tensile strength was first reduced and then hold above 1400 MPa. In addition, the microstructure of the hot stamping parts was lath martensite, and martensite lath length and packet width increases with the heating temperature increased from 850 °C to 1050 °C. Especially, the effect of heat temperature on the original austenite grain size was more obvious, such as the austenite grains grew up quickly with the increase of heating temperature, and the original austenite grain diameter was 37.8 μm when the temperature reached 1050 °C.

Flow Stress Behaviors and Microstructure Evolution of 300M High Strength Steel under Isothermal Compression

The compressive deformation behaviors of 300M high strength steel were investigated over a wide range of temperatures (850–1200 °C) and strain rates (0.001–10s−1) on a Gleeble-3800 thermo-mechanical simulator. The measured flow stress was modified by the corrections of the friction and the temperature compensations, which nicely reflect negative effects of the friction and temperature on the flow stress. The corrected stress-strain curves were the dynamic recrystallization type on the conditions of higher deformation temperature and lower strain rate. Flow stress increases with the increase of strain rate at the same deformation temperature and strain. By contrast, flow stress decreases with the increase of temperature at the same strain rate and strain. Dependence of the peak stress on temperature and strain rate for 300M steel is described by means of the conventional hyperbolic sine equation. By regression analysis, the activation energy (Q) in the whole range of deformation temperature is determined to be 367.562 kJ/mol. The effects of the temperature and the strain rate on microstructural evolution are obvious. With the increase of the deformation temperature and the decrease of the strain rate, the original austenite grain sizes of 300M steel increase. At the same time, the corrected flow stress curves more accurately determine the evolution of the microstructure.

Effect of Laser Welding Process Parameters on Microstructure and Mechanical Properties on Butt Joint of New Hot-Rolled Nano-scale Precipitation-Strengthened Steel

A 4 kW fiber laser was chosen to weld the new hot-rolled nano-scale precipitation-strengthened steel with a thickness of 4.5 mm. The effect of laser power, defocusing distance, and welding speed on the welded joint appearance was examined, and the microstructure and mechanical properties on the typical butt joints were investigated. Results showed that increasing laser welding power may cause faster downward flow of molten metal to produce greater root humping. With the welding speed increasing, the average welding seam (WS) width decreased, and the average WS and heat-affected zone (HAZ) hardness increased. The microstructures of WS, fusion line, and coarse grain heat-affected zone were lath martensite, but the growth direction of the original austenite grain boundaries was significantly different. The microstructures of fine grain heat-affected zone were ferrite and martensite, and the microstructure of mixed grain heat-affected zone contained ferrite, massive M/A island, and a small amount of martensite. The micro-hardness values of WS, HAZ, and base metal (BM) were 358, 302, and 265 HV, respectively. The butt joint fracture at the BM far from the WS and the welded joint tensile strength are observed to follow proportional relationship with hardness.