Fine Prior Austenite Grain Research Articles

Effect of Initial Intergranular Ferrite Size on Induction Hardening Microstructure of Microalloyed Steel 38MnVS6

In this study, we investigated the effect of grain size of an initial microstructure (pearlite + ferrite) on a resulting microstructure of induction-hardened microalloyed steel 38MnVS6, which is one topical medium carbon vanadium microalloyed non-quenched and tempered steel used in manufacturing crankshafts for high-power engines. The results show that a coarse initial microstructure could contribute to the incomplete transformation of pearlite + ferrite into austenite in reaustenitization transformation by rapid heating, and the undissolved ferrite remains and locates between the neighboring prior austenite grains after the induction-hardening process. As the coarseness level of the initial microstructure increases from 102 μm to 156 μm, the morphology of undissolved ferrite varies as granule, film, semi-network, and network, in sequence. The undissolved ferrite structures have a thickness of 250–500 nm and appear dark under an optical metallographic view field. To achieve better engineering applications, it is not recommended to eliminate the undissolved ferrite by increasing much heating time for samples with coarser initial microstructures. It is better to achieve a fine original microstructure before the induction-hardening process. For example, microalloying addition of vanadium and titanium plays a role of metallurgical grain refinement via intragranular ferrite nucleation on more sites, and the heating temperature and time of the forging process should be strictly controlled to ensure the existence of fine prior austenite grains before subsequent isothermal phase transformation to pearlite + ferrite.

Characterization of microstructural evolution with pre-strain in BG801 bearing steel: Grain, carbides, retained austenite and martensite

The method of multiple cryogenic and tempering treatments significantly limits the microstructure regulation of BG801 bearing steel. In this work, the pre-strain was introduced in the vacuum-melted BG801 steel combing with heat treatment (HT) to adjust the microstructures. The samples were subjected to uniaxial tensile at room temperature with different strains (engineering strain of 0, 0.15, 0.175, 0.2), then heating to 1323 K and water-cooling. The results showed that higher pre-strain was beneficial to obtain finer prior austenite grain (PAG) and martensite grains, and a low fraction of retained austenite (RA) in BG801 steel. With increasing pre-strain, the PAG was elongated along the RD direction (rolling direction) without a change in size. The recrystallized grains with a size of about 5–6 μm nucleated inside the PAG during HT. Meanwhile, the high-density dislocations were introduced in the tensile samples by pre-strain, promoting the growth of micron-M6C by accelerating the diffusion of alloy elements during HT. And the evolution of RA and martensite indicated that the pre-strain accelerates transformation from austenite to martensite by transferring elements from the matrix to M6C and introducing shear bands into the matrix to promote the nucleation of martensite.

DT300 ultra-high strength steel fabricated using selective laser melting: Densification, microstructure, and mechanical properties

DT300 ultra-high strength steel has been widely used due to its combination of high mechanical properties and a relative low cost. However, the traditional casting and forging process can hardly meet the rising needs of producing parts with complicated shapes and high precision. In this study, selective laser melting was utilized to fabricate DT300 steel for the first time. Fully dense (>99% relative density) parts were successfully fabricated and the types and formation mechanism of defects under different conditions are discussed in detail. The average equivalent diameter of martensite laths is only 0.64 μm, and the retained austenite content of horizontal and vertical section is 5.6% and 7.1% in volume fraction, respectively. The fine prior austenite grain boundaries are continuously and uniformly distributed both in horizontal and vertical sections. Both martensite and austenite show strong texture orientation, which is closely related to the laser scanning direction of each layer. The specimens fabricated by the optimized parameter M8 (250 W laser power, 800 mm/s laser scanning speed, 90 μm hatch spacing) possess outstanding mechanical properties (yield strength up to 1309 MPa, ultimate tensile strength up to 1636 MPa, 19.79% elongation to fracture) as well as outstanding surface quality (Ra of 14.37 ± 2.15 μm). The mechanical strength of SLM fabricated DT300 steel is higher than that of as-cast with heat treatment condition but 10% lower than the wrought one with post heat treatment.

Effect of Ca Deoxidation on Toughening of Heat-Affected Zone in High-Strength Low-Alloy Steels after Large-Heat-Input Welding

Large-heat-input welding can effectively increase the efficiency and reduce the cost of manufacturing a super-large container ship for marine trade worldwide with thick, high-strength low-alloy (HSLA) steel plates; however, it significantly degrades the toughness of the welding heat-affected zone (HAZ). This paper describes the effect of Ca deoxidation on the impact toughness of simulated coarse-grained HAZs (CGHAZs) in HSLA steels after large-heat-input welding at 400 kJ cm−1. The average impact energy of the CGHAZ increases with an increase in Ca content; in particular, the energy of the steel with 25 ppm Ca content is satisfactorily high, owing to the uniform and fine prior austenite grains. In contrast, the grains in the CGHAZs of the steels with relatively low Ca contents are not uniform, leading to large test variabilities at −20 °C. Failure analyses reveal that the major and secondary cracks preferentially propagate along the coarse and brittle grain boundary ferrite (GBF), but their propagation is retarded by the fine and interlocking intragranular acicular ferrite (IAF) as the grain size decreases. It is therefore recommended to increase the Ca content to about 25 ppm during the deoxidation of steelmaking to improve HAZ toughness by forming fine and uniform prior austenite grains and IAF within grains.

Inclusion and mechanical properties of ODS-RAFM steels with Y, Ti, and Zr fabricated by melting

Two groups of oxide dispersion–strengthened reduced-activation ferritic/martensitic steels (A and B) were prepared by adding Y, Ti, and Zr into steels through vacuum induction melting to investigate the inclusions, microstructures, mechanical properties of the alloys. Results showed that particles with Y, Ti, and Zr easily formed. Massive, Zr-rich inclusions were found in B steel. Density of micron inclusions in A steel was 1.42 × 1014 m−3, and density of nanoparticles was 3.61 × 1016 m−3. More and finer MX carbides were found in steel tempered at 650 °C, and yield strengths (YS) of A and B steel were 714±2 and 664±3.5 MPa. Thermomechanical processing (TMP) retained many dislocations, which improved the mechanical properties. YSs of A and B treated by TMP were 725±3 and 683±4 MPa. The existence of massive Zr-rich inclusions in B steels interrupted the continuity of the matrix and produced microcracks (fracture), which caused a reduction in mechanical properties. The presence of fine prior austenite grain size and inclusions was attributed to the low DBTTs of the A steels; DBTTs of A650 and A700 alloy were −79 and −65 °C. Tempering temperature reduction and TMP are simple, readily useable methods that can lead to a superior balance of strength and impact toughness in industry applications.

Role of Vanadium Carbide in Hydrogen Embrittlement of Press-Hardened Steels: Strategy From 1500 to 2000 MPa

This work investigated hydrogen trapping and hydrogen embrittlement (HE) in two press-hardened steels, 22MnB5 for 1,500 MPa grade and 34MnB5V for 2000 MPa grade, respectively. Superior to the 22MnB5 steel, the newly developed 34MnB5V steel has an ultimate tensile strength of over 2000 MPa without sacrificing ductility due to the formation of vanadium carbides (VCs). Simulated press hardening was applied to two steels, and hydrogen was induced by cathodic charging. Susceptibility to HE was validated by slow strain-rate tensile test. When hydrogen content was high, the 34MnB5V steel fractured in elastic regime. However, when hydrogen content was 0.8–1.0 ppmw, the 34MnB5V steel bore much higher stress than the 22MnB5 steel before fracture. The behavior of hydrogen trapping was investigated by thermal desorption analyses. Although the two steels trapped similar amounts of hydrogen after cathodic charging, their trapping mechanisms and effective trapping sites were different. In summary, a finer prior austenite grain size due to the pinning effect of VCs can also improve the toughness of 34MnB5V steel. Moreover, trapping hydrogen by grain boundary suppresses the occurrence of hydrogen-enhanced local plasticity. Microstructural refinement enhanced by VCs improves the resistance to HE in 34MnB5V steel. Importantly, the correlation between hydrogen trapping by VCs and improvement of HE is not significant. Hence, this work presents the challenge in designing irreversible trapping sites in future press-hardened steels.

Comparison of microstructure and heat treatment distortion of gear steels with and without Nb addition

In order to research the effect of microalloying Nb, the microstructure and heat treatment distortion of case carburizing steels with and without Nb addition are compared. Results show that a uniform and fine prior austenite grain size was obtained for the steel with the addition of 0.03 wt.% Nb even after being carburized at 980 °C for 37 h, resulting in a very deep hardened layer of about 4 mm. Nb is an effective microalloying element to hinder austenite grain growth of gears during high-temperature carburizing. Theoretical calculation and experimental observation of NbC precipitates indicate that fine NbC precipitates have not evidently dissolved at 980 °C, and thus they can act as grain refiners due to pinning effect. Heat treatment distortion of Nb-added steel is much lower than that of the steel without Nb addition. It may be contributed to its fine and uniform grain size, which presumably influences stress during martensitic transformation.

2.47 GPa grade ultra-strong 15Co-12Ni secondary hardening steel with superior ductility and fracture toughness

This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15Co-12Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain (PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness (KIC) was 29.3 MPa m, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode (cup-and-cone) and the formation of about ∼1 μm ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples. The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution, deformation and fracture mechanisms along the heat-treatment was also discussed.

Effects of Nitrogen Content and Hot Forming Temperature on Prior Austenite Grain Size and Mechanical Properties for Normalizing Cryogenic Pressure-Vessel Steels

The effects of nitrogen content and hot forming temperature on prior austenite grain size for normalizing cryogenic pressure-vessel steels were described. The evolution of V(C,N) precipitates during thermal cycle of hot forming was analyzed. The optical microscopy displays that as increasing N content from 0.0094 to 0.0198 wt.% the refinement of prior austenite grain was seen at each hot forming temperature of 870, 910 and 950 °C. The transmission electron microscopy presents that much finer V(C,N) precipitates were found in sample with 0.0198 wt.% N content than in sample with 0.0094 wt.% N content. Thermo-Calc calculation reveals the precipitation temperature of V(C,N) was increased by increasing N content. While the Ostwald ripening rate of V(C,N) was decreased by increasing N content. TEM result and Thermo-Calc calculation indicate that increasing N content enhanced the thermal stability of V(C,N) precipitates. The stable finer V(C,N) precipitates contribute to the finer prior austenite grain and good mechanical properties for sample with 0.0198 wt.% N content even at higher temperature of 950 °C during hot forming process.

Effects of Alloying Elements Addition on Delayed Fracture Properties of Ultra High-Strength TRIP-Aided Martensitic Steels

To develop ultra high-strength cold stamping steels for automobile frame parts, the effects of alloying elements on hydrogen embrittlement properties of ultra high-strength low alloy transformation induced plasticity (TRIP)-aided steels with a martensite matrix (TM steels) were investigated using the four-point bending test and conventional strain rate tensile test (CSRT). Hydrogen embrittlement properties of the TM steels were improved by the alloying addition. Particularly, 1.0 mass% chromium added TM steel indicated excellent hydrogen embrittlement resistance. This effect was attributed to (1) the decrease in the diffusible hydrogen concentration at the uniform and fine prior austenite grain and packet, block, and lath boundaries; (2) the suppression of hydrogen trapping at martensite matrix/cementite interfaces owing to the suppression of precipitation of cementite at the coarse martensite lath matrix; and (3) the suppression of the hydrogen diffusion to the crack initiation sites owing to the high stability of retained austenite because of the existence of retained austenite in a large amount of the martensite–austenite constituent (M–A) phase in the TM steels containing 1.0 mass% chromium.

Martensitic Steel Microstructure Effects on Cavitation Erosion

Abstract Cavitation and cavitation erosion are sometimes unavoidable phenomena in hydro machine operation. For example, operating hydro turbines as regulating power leads to situations in which the risk of cavitation is accepted to some extent. Cavitation-resistant materials are therefore required to reduce machine damage and maintenance. This study characterizes the microstructure of two martensitic stainless steels from Francis turbines. They were already studied for cavitation erosion resistance in a previous study, and this study reveals the reasons behind the other steel having a significantly better resistance, while they had similar chemical compositions. The electron backscatter diffraction (EBSD) method is effective in defining the martensitic microstructure, specifically the block, packet, and prior austenite grain level. In addition, the retained austenite is effectively detected with the method. A fine prior austenite grain size and small packet and block sizes were found to be among the defining factors in cavitation erosion resistance of these steels. In addition, the transformation of retained austenite to martensite was detected in the edge region where cavitation had taken place. This transformation further increases cavitation erosion resistance. The better resistance of Steel 1 against cavitation was attributed to these microstructural differences. According to these findings, the microstructure in Steel 1 would be highly beneficial in building cavitation erosion resistant hydro machines and would be of interest to manufacturers of martensitic stainless steel components.

Effects of Ti Addition on the Microstructure and Tensile Properties of China Low Activation Martensitic Steel for Nuclear Fusion Reactors

Herein, the effects of Ti, a low activation alloying element, in the range of 0.01–0.11 wt% on the microstructure, mechanical properties, and impact toughness of China low activation martensitic steel are studied. Fine prior austenite grains and some δ‐ferrite phases are observed for alloys with a Ti content of 0.05 and 0.11 wt%, respectively. The 0.05Ti alloy exhibits a higher yield strength (YS) compared with the 0.01Ti and 0.11Ti alloys because of the additional precipitation hardening by the fine MX particles and small grain size. When the samples are tempered at 650 °C, nanosized (Ti, W) C MX carbides with a size of approximately 20 nm precipitate, leading to an increase in the ultimate tensile strength (UTS) and YS of the 0.05Ti alloy. The ductile‐to‐brittle transition temperature (DBTT) decreases when the Ti content increases to 0.05 wt%. However, the DBTT of the 0.11Ti alloy increases with the precipitation of the δ‐ferrite phase. On the basis of the performance of the steels, the optimal Ti content is found to be approximately 0.05 wt% and the optimal tempering temperature is found to be 650 °C.

Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, microstructures, macrohardness and microhardness, continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities. Cooling rates (CRs) of 60, 1 and 0.01 °C/s were selected for more detailed investigations. Microstructural characterization was made by laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, electron probe microanalysis and X-ray diffraction. Mechanical properties were characterized using macrohardness, tensile and Charpy V-notch impact tests. UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content. Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size, as well as effective final grain size and lath size. These changes were accompanied by higher yield and tensile strengths. The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR. For the cooling rates studied, UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.

On the heterogeneous microstructure development in the welded joint of 12MnNiVR pressure vessel steel subjected to high heat input electrogas welding

Microstructure features of 12MnNiVR pressure vessel steel welded joint deposited by the high heat input electrogas welding have been systematically investigated. It is revealed that the welded joint is featured by a heterogeneous juxtaposition. The coarse grained heat-affected zone (CGHAZ) primarily consists of lath bainites and minor granular bainites. The fine grained heat-affected zone (FGHAZ) is dominated by polygonal ferrites, pearlites, and fine cementite particles. Moreover, electron backscatter diffraction results further demonstrate that the CGHAZ is populated by coarse prior austenite grains (PAGs) with high frequency (61.3%) of low angle grain boundaries (LAGBs). On the other hand, the FGHAZ is filled with fine PAGs with a lower frequency (19.6%) of LAGBs. Such microstructural differences may likely contribute to differed mechanical properties for samples tested at designed positions.

Analysis of creep behavior of welded joints of P91 steel at 600 °C

The martensitic P91 steel (9Cr-1Mo-V-Nb) has been developed for ultra-supercritical pressure application in steam power plants. The creep rupture strength of the welded joint of this steel is limited by the fine-grained region of its heat affected zone (HAZ). The factors which result in the reduction of the creep rupture strength of the fine-grained region of HAZ were discussed. It was found that the most effective factor reducing the creep rupture strength of the fine-grained region of the HAZ of the welded joint of P91 steel, in comparison with other regions of the welded joint, is the finer prior austenite grain size. These fine prior austenite grains of the fine-grained region of the HAZ accelerate the rate of growth of martensite lath subgrains which results in a softer martensite matrix.The minimum creep rate dependence on applied stress for base metal, welded joints and the samples simulating the fine-grained region of the welded joints was described by means of the conventional power-law equation. The experimental results, both in the form of strain versus time creep curves and of minimum creep rate versus time creep curves were used to study the creep behavior of the welded joints of P91 steel at 600 °C.

Reheat Furnace Thermodynamic, Kinetic and Combustion Considerations for TMCP Processing

The reheat furnace process step has a profound effect on the TMCP performance, final hot rolled steel quality and mechanical property consistency during the production of hot rolled steels. The uniformity of heating applied across the entire width and length of the slab or billet is critical in the achievement of customer properties regardless of the chemistry. The resultant ferrite grain size in the final hot rolled product is significantly governed by the initial prior austenite grain size. Numerous reheat furnace process metallurgy and combustion parameters in actual operation affect mill productivity, microstructure, austenite grain size, scrap rate and diverts. This reheating step in the steelmaking process often receives low priority in the evaluation of product quality and mechanical property performance, especially the toughness through the plate thickness. Heat transfer conditions of radiation, convection and conduction affect furnace heating efficiency. In laboratory studies, the furnace heating step is typically quite uniform resulting in a homogeneous and fine prior austenite grain size. During production, it is much more difficult to control the uniformity of heating and heat transfer consistency along the entire length and through the thickness of the work piece. The furnace conditions are correlated to product quality via furnace process variables such as the air to gas ratio, furnace burner condition, furnace pressure, energy efficiency, adiabatic flame temperature (AFT) and furnace refractory condition. Operational practice recommendations are presented to minimize inhomogeneous heating which results in inferior product quality, hot rolling model anomalies and toughness variations in the through-thickness-direction.

Effect of Nb micro-alloying on microstructure and properties of thermo-mechanically processed high carbon pearlitic steel

Two C-Mn-Si steels with and without Nb micro-alloying are selected in the present study. Both these high carbon steels have been thermo-mechanically processed using Gleeble 3800 simulator. Optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM) were utilised for microstructural characterisation. The austenitising temperature was varied from 1150 °C to 1200 °C followed by hot compression and subjected to free cooling to evaluate the influence of austenite grain size on transformed microstructure and mechanical properties. It is evident that higher austenite grain size is obtained for both types of steels subjected to higher austenitising temperature. The refinement of pearlite interlamellar spacing (<0.20 μm) with degenerated morphology as well as finer prior austenite grain size (<40 μm) is found to be more prominent for Nb micro-alloyed sample. The average hardness values of both the steels are higher for the specimens, treated at a higher austenitising temperature which is attributed to finer interlamellar spacing. Finally, the correlation between the evolving microstructure and resulting mechanical properties (hardness and yield strength) has been made.

Design of Online Spheroidization Process for 1.0C-1.5Cr Bearing Steel and Microstructure Analysis

Using thermo-mechanical control process, the online spheroidization annealing process of 1.0C-1.5Cr bearing steel was designed. Apart from intercritical online spheroidization (IS), a novel subcritical online spheroidization (SS) process was proposed, which is characterized by water-cooling to around 773 K (500 °C) after the final rolling pass, and then directly reheating to 973 K (700 °C) for isothermal holding. Compared with the results from the traditional offline spheroidization (TS) process, the size of spheroidized carbides is similar in both the TS and IS processes, whereas it is much smaller in the SS process. After spheroidization annealing, microstructure evolution during austenitization and quenching treatment was examined. It is shown that the refining of spheroidized carbides accelerates the dissolution of carbides during the austenitizing process, and decreases the size of undissolved carbides. In addition, the SS process can obtain finer prior austenite grain after quenching, which contributes to the enhancement of final hardness.

Effect of Weld Thermal Cycles on Microstructures and Mechanical Properties in Simulated Heat Affected Zone of a HY 85 Steel

Physical weld simulation for single and two-pass weld of a HY 85 steel has been done using thermo mechanical simulator at constant heat input of 22 kJ/cm. Attempt has been made to investigate the causes behind the deterioration of mechanical properties in the coarse grain heat affected zone (CGHAZ) in single pass and subsequent improvement in the mechanical properties of CGHAZ region in the two-pass weld. HY 85 steel finds wide applications in the making of ship hull. Impact toughness at −50 °C for CGHAZ in the single pass weld has been found to be 49 J. However, impact toughness improves to 122 J in the super critical reheated CGHAZ region for two-pass weld. This improvement in the impact toughness has been observed due to reaustenitization of fine prior austenite grains of average size of 44 µm from 99 µm, refinement of bainitic ferrite lath, and M–A constituents during second pass weld.

Effect of Normalization Heat Treatment on Creep and Tensile Properties of Modified 9Cr–1Mo Steel

Creep and tensile properties have been investigated on modified 9Cr–1Mo steel subjected to single and double normalization heat treatments. Optical, scanning and transmission electron microscopic investigations revealed the presence of finer prior austenite grain size and coarsening of MX precipitates in the double normalized steel as compared to the steel subjected to single normalization heat treatment. Increased minimum creep rate and significant reduction in creep rupture life were observed in the double normalized steel. Increased tensile ductility coupled with marginal decrease in tensile strength was observed with double normalized steel compared to single normalized steel. The double normalized steel exhibited lower creep strength and higher tensile ductility than the single normalized steel.