Effect of Reversed Austenite on Mechanical Properties of ZG06Cr13Ni4Mo Repair Welded Joint

In this work, gas tungsten arc welding (GTAW) was used to repair ZG06Cr13Ni4Mo martensitic stainless steel. Repair welding occurred either once or twice. The changes in the microstructure and properties of the repair welded joints were characterized by optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), tensile and impact tests. The effects of reversed austenite in repair welded joints on microstructure and mechanical properties were studied. The results show that the microstructure of the welded joint after repair welding consists of a large amount of martensite (M) and a small amount of reversed austenite (A), and the reversed austenite is distributed at the boundary of martensite lath in fine strips. With the increase in the number of welding repairs, the content of reversed austenite in the welded joint increases. The microstructure in the repair welded joints is gradually refined, the microstructure in the once and twice repaired joints is 45.2% and 65.1% finer than that in the casting base metal, respectively. The reversed austenite presented in the repair welded joints decreases the tensile strength by 4.8% and 6.7%, increases the yield strength by 21.3% and 26.4%, and increases the elongation by 25% and 56%, respectively, compared with the casting base metal. In addition, the reversed austenite mainly nucleates and grows at the boundary of lath martensite. The refinement of the martensite structure was due to the generation of reversed austenite and the refinement of original austenite grain by the welding thermal cycle. After repair welding, the reverse austenite appeared in the repair welded joints and the tensile strength decreased slightly, but the plastic toughness was significantly improved, which was conducive to the subsequent service process.

Effect of Deformation on Microstructure and Mechanical Properties of Medium Carbon Steel During Heat Treatment Process

The current research of the Q-P and Q-P-T process has been focused on controlling the heating temperature and holding time, or adding alloy elements into the steel to induce precipitation strengthening and improve the strength and plasticity of the steel. In this article, based on a quenching-partitioning-tempering (Q-P-T) process combined with a hot deformation technology, a deforming-quenching-partitioning-tempering (D-Q-P-T) process was applied to medium carbon steel. The effect of the heat treatment parameters on the microstructure and mechanical properties of experimental steel under deformation was studied. Through use of a scanning electron microscope (SEM), transmission electron microscopy (TEM) and tensile tests, the optimal heat treatment conditions for realizing high strength and plasticity that meet the safety requirements were obtained. The mechanism for the D-Q-P-T process to improve the strength and plasticity of experimental steel was discussed. A multiphase composite structure of lath martensite and retained austenite was obtained. Compared with the Q-P-T process, use of the D-Q-P-T process can increase the strength of steel by 57.77 MPa and the elongation by 5%. This study proposes a method to improve the strength and plasticity of steel.

Study on the hydrogen-induced delayed fracture behavior of Q-P980 and MS980

In this paper, the hydrogen diffusion behavior and hydrogen induced delayed fracture (HIDF) of Q-P980 (Q-P: Quenching and Partitioning) and MS980 (MS: Martensitic steel) steels were investigated using hydrogen penetration, slow strain rate tensile (SSRT) tests, thermal desorption spectroscopy (TDS) tests, fracture analysis, and microstructural examination in this paper. The austenite in Q-P980 is massive retained-austenite (RA) with low stability. The TRIP (Transformation Induced Plasticity) effect will occur in the process of strain and change into high carbon martensite. HIDF is caused by a substantial amount of surplus hydrogen being enriched at the border and flaws. The fracture has a broad cleavage surface and is a typical quasi-cleavage fracture. MS980 has been sufficiently tempered, resulting in a substantial quantity of distributed spherical cementite (150nm) precipitating around the lath martensite. This size and form of cementite may successfully trap hydrogen while maintaining the material's mechanical characteristics. And tempering can effectively reduce the local stress level of steel, so MS980 has a very low HE susceptibility. HIDF is related to local stress and hydrogen accumulation. We suppose that Z is a constant and ZC is a critical value which associated to σ and CH (the local stress and local hydrogen concentration), rising as σ and CH rises. The atomic bonds at the crack tip, lattice position and the phase interface will fracture when ZC reaches a particular value Z. Tempering to minimize local stress and carbide precipitation to capture hydrogen are two strategies for reducing hydrogen embrittlement (HE) susceptibility, particularly for dislocation strengthened steel. Microalloying elements can generate precipitates that function as hydrogen traps and obstruct the HELP (Hydrogen Enhanced Localized Plasticity) process, lowering local stress and hydrogen accumulation.

Effect of Cooling Rate on The Microstructure And Cryogenic Impact Toughness of HAZ in 9% Ni Steel

The effects of cooling rate on the microstructure and cryogenic impact toughness of coarse-grained heat-affected zone (CGHAZ) and inter-critically reheated coarse-grained HAZ (IC CGHAZ) in 9% Ni steel were investigated. CGHAZ and IC CGHAZ specimens were prepared from 9% Ni steel by controlling the cooling rate of the simulated welding process. The microstructure of the CGHAZ specimens consisted of autotempered martensite and lath martensite. As the cooling rate increased, the volume fraction of the autotempered martensite and the effective grain size decreased. A large amount of fine carbides was distributed inside the auto-tempered martensite, the dislocation density was low, and high angle grain boundaries were not observed. The microstructure of the IC CGHAZ specimens consisted of tempered martensite and lath martensite. As the cooling rate increased, the volume fraction of the tempered martensite and effective grain size decreased. Finer carbides were distributed inside the tempered martensite than in the auto-tempered martensite, the dislocation density was low, and high angle grain boundaries were not observed. Cryogenic fracture revealed that ductile fracture occurred in the auto-tempered martensite and tempered martensite, and brittle fracture occurred in the lath martensite. The crack propagation path was zig-zag in the high angle grain boundaries of the lath martensite. The volume fraction of auto-tempered martensite and tempered martensite and the effective grain size in the HAZ specimens had a significant effect on cryogenic impact toughness. In the IC CGHAZ specimens, cryogenic impact toughness decreased and then became constant as the cooling rate increased, due to a decrease in the volume fraction of the tempered martensite and effective grain size.

Small Diameter Tube Post-Weld Heat Treatment Process with Brinell Hardness and Microstructure Inspection

The local post-weld heat treatment (PWHT) process test of 9Cr-3W-3Co small-diameter pipe welded joints was carried out using rope-type resistance heaters with different parameters, and the Brinell hardness test and microstructure observation were carried out on the welded joints after heat treatment. The results show that when the heating width was 200mm, the constant temperature was 790°C-800°C, and the constant temperature time was 2 hours, the Brinell hardness of the weld was in the range of 246HBW-265HBW, which had good performance. After tempering at 760°C-800°C, the welds all showed a clear tempered lath martensite.

Numerical Simulation and Microstructure and Properties of 45 Steel by Scanning Electron Beam Bevel Polishing

In order to obtain metal products with good surface quality, it is necessary to develop an efficient bevel polishing method. Therefore, this article Uses continuous scanning electron beam polishing. The surface of the material undergoes rapid melting and solidification and generates a dynamic temperature field. The polishing treatment improves the microstructure of the surface layer of the quenched and tempered 45 steel, and significantly increases the microhardness of the surface layer. The hardness after treatment can reach up to 747.6Hv, which is about 2.4 times higher than the matrix; the structure after hardening is transformed from the mixed phase of coarse acicular martensite and lath martensite to hidden acicular martensite and retained austenite a mixed phase thereof. In addition, after bevels with different inclination angles are subjected to electron beam surface polishing modification treatments with different scanning speeds, under the same parameters, as the inclination angle increases, the hardness value at the same position decreases.

Effect of laser cladding on microstructural transformation and mechanical properties of heat affected zone of EA4T steel

In this study, laser cladding on EA4T steel was carried out. The Composition of HAZ of the EA4T steel was analyzed by OM and SEM. The effect of laser cladding on microstructural transformation of HAZ of EA4T steel was discussed. The distribution of alloying elements and the micromechanical properties of the interface were tested. The results show that the microstructures of the laser cladding heat-affected zone of the EA4T steel are mainly lath martensite. The homogenization of austenite and the dissolution of carbides near the interface are not sufficiently developed, and coarse grains are observed. In the fine-grained area of the HAZ, the microstructure is mainly composed of martensite and a small amount of sorbite with a certain orientation. In addition, among all laser cladding conditions, the highest temperature experienced in the HAZ of the EA4T steel plays a decisive role in the evolution of the microstructure and mechanical properties. A sudden change in alloying elements occurs at the junction site, and the grain orientation is clearly found. Moreover, the shear strength of the HAZ increases, whereas the degree of packing decreases.

Microstructure and mechanical properties of 9Cr-3W-3Co steel welded joints by vacuum electron beam welding

This paper aims to explore the microstructure and mechanical properties of 9Cr-3W-3Co steel welded joints. In the experiment, 9Cr-3W-3Co steel samples were welded by vacuum electron beam welding technology (VEBW) without any metal stuff, and all the welding joints were treated by high-temperature tempering at [Formula: see text]C for 8 h. The microstructure of welding joints was observed by OM, SEM and TEM; and the mechanical properties of welded joints were analyzed by microhardness test, room-temperature tensile, test impact test and high-temperature creep test. As a result, all the 9Cr-3W-3Co steel samples displayed the microstructure status as martensite under the Scheffler-Schneider prediction model, which conformed to the expectation. After high-temperature tempering, the grains of the welding zone were smaller than the base metal and the composition was tempered lath martensite only. Some of the lath martensite bundles even showed the incomplete polygonal transformation. The M[Formula: see text]C6 carbides and MX phase were distributed continuously along with the lath martensite interfaces, which showed a tendency for further aggregation. The microhardness of the weld zone was slightly higher than the base metal (mean of base metal: 240 HV[Formula: see text], mean of weld zone: 273 HV[Formula: see text] and mean of heat affected area: 274 HV[Formula: see text]. There was no softening phenomenon observed, and the welding joints maintaining the high intensity. Other mechanical properties like the tensile strength (mean: 750 MPa), yield strength (mean: 707 MPa) and impact toughness (mean of WM: 25.1 J and HAZ: 23.3 J) were also excellent. When the temperature parameter is constant, the time for creep fracture reduces significantly with the increase of the stress; whereas the time for creep fracture decreases significantly as the temperature increases, while the stress parameter is constant.