Showing papers on "Tempering published in 2018"

Homogenization of P91 weldments using varying normalizing and tempering treatment

Abstract: Creep strength enhanced ferritic/martensitic P91 steel is considered as a candidate material for the reactor pressure vessels and reactor internals of Very High Temperature Reactor (VHTR). Heterogeneous microstructure formation across the P91 weldments lead to premature Type IV cracking and makes the weldability of P91 steel a serious issue. The present research work describes the effect of normalizing and tempering (N&T) treatment on microstructure evolution in various zones of gas tungsten arc welded (GTAW) P91 pipe weldments. For N&T treatment, P91 pipe weldments were subjected to various normalizing (950–1150 °C) and tempering (730–800 °C) temperature. The effect of varying heat treatment on tensile properties and hardness of P91 pipe weldments were studied for V-groove and narrow-groove weld designs. The effect of increase in normalizing temperature (fixed tempering temperature) resulted in increase in strength and hardness, while increase in tempering temperature (fixed normalizing temperature) resulted in the decrease in strength and hardness of P91 steel weldments. The better combination of strength, ductility and microstructure were obtained for the maximum normalizing temperature of 1050 °C and tempering temperature of 760 °C.

The influence of post-production heat treatment on the multi-directional properties of nickel-aluminum bronze alloy fabricated using wire-arc additive manufacturing process

Abstract: In this paper, a nickel-aluminum bronze alloy component is built using wire-arc additive manufacturing process. In order to investigate the influence of anisotropy introduced by the wire-arc additive manufacturing process, the layer-by-layer manufactured components with different post-production heat treatments are characterized by optical and scanning electron microscopy morphologies, X-ray diffraction and mechanical tests in longitudinal, transverse and normal directions. The experimental results show that the deposit exhibits higher strengths in the longitudinal and transverse direction than in the normal direction. Also, the ductility of the alloy is significantly improved with the designed quenching and tempering method, and competitive mechanical properties are achieved when tempering temperature reaches 650 °C. In addition, the anisotropy in the additively manufactured alloy can be effectively modified by the quenching and tempering heat treatments.

Prediction of microstructure, residual stress, and deformation in laser powder bed fusion process

Abstract: Laser powder bed fusion (L-PBF) process has been investigated significantly to build production parts with a complex shape. Modeling tools, which can be used in a part level, are essential to allow engineers to fine tune the shape design and process parameters for additive manufacturing. This study focuses on developing modeling methods to predict microstructure, hardness, residual stress, and deformation in large L-PBF built parts. A transient sequentially coupled thermal and metallurgical analysis method was developed to predict microstructure and hardness on L-PBF built high-strength, low-alloy steel parts. A moving heat-source model was used in this analysis to accurately predict the temperature history. A kinetics based model which was developed to predict microstructure in the heat-affected zone of a welded joint was extended to predict the microstructure and hardness in an L-PBF build by inputting the predicted temperature history. The tempering effect resulting from the following built layers on the current-layer microstructural phases were modeled, which is the key to predict the final hardness correctly. It was also found that the top layers of a build part have higher hardness because of the lack of the tempering effect. A sequentially coupled thermal and mechanical analysis method was developed to predict residual stress and deformation for an L-PBF build part. It was found that a line-heating model is not suitable for analyzing a large L-PBF built part. The layer heating method is a potential method for analyzing a large L-PBF built part. The experiment was conducted to validate the model predictions.

Radio frequency tempering uniformity investigation of frozen beef with various shapes and sizes

Abstract: Radio frequency (RF) energy generates fast and volumetric heating as it penetrates food materials and converts electromagnetic energy to heat. With these advantages, RF heating is considered as a promising technology for tempering and thawing processes in the meat and fishery products industry. However, non-uniform heating problems hinder its further application to meat products due to their various sizes and irregular shapes. This study utilized representative frozen beef samples to investigate the parameters of varying sample thickness (40 mm; 50 mm; 60 mm), base area (small:160 × 102 × 60 mm3; medium: 220 × 140 × 60 mm3; large: 285 × 190 × 60 mm3) and shape (cuboid; trapezoidal prism; step) and their influence on tempering uniformity in a parallel-plate RF system. A computer simulation model was established, verified by experiments and then was utilized to evaluate the volumetric temperature distribution in food samples. Results show that the heating rate increases and heating uniformity decreases with increasing sample thickness and decreasing sample base area. As sample thickness increased from 4 cm, 5 cm to 6 cm, the simulated temperature uniformity index (STUI) increased from 0.093, 0.117 to 0.194. Sample base area increases from small to large decreased the STUI from 0.229 to 0.194 and 0.090. Among all three shapes, the cuboid shape has the best heating uniformity (STUI 0.194), followed by the trapezoidal prism (STUI 0.209) and the step shape (STUI 0.282). The step shape has the worst tempering uniformity because the RF energy focuses mainly on the vertical section and results in severe regional heating. Strategies to improve the step-shape frozen beef tempering uniformity by decreasing the input power to 1/3 and enlarging the electrode gap by 40 mm only reduced the hot spot temperature from 88 to 78 °C. Further research is needed in order to develop methodologies or suitable equipment for irregular shape food RF tempering in the future. Industrial relevance In industrial radio frequency thawing/tempering, the raw materials are usually presented in various irregular shapes and sizes. Thus, analyzing the non-uniformity severity influenced by sample size, shape and thickness to determine the capability and throughput of the equipment is necessary. Results in this study could be utilized in pre-evaluation of a protocol design and process optimization for irregular-shape food tempering.

Effects of deep cryogenic treatment on microstructural evolution and alloy phases precipitation of a new low carbon martensitic stainless bearing steel during aging

Abstract: It is evident from controversial statements in the literature that the effects of deep cryogenic treatment on the microstructural evolution of low carbon martensitic steels are not fully understood yet An investigation has been started to clarify this situation by analyzing the microstructural evolution and alloy precipitation after aging The results show that the hardness is enhanced after deep cryogenic treatment and this is mainly contributed to the transformation of retained austenite to martensite at low temperature By comparing with the samples subjected to quenching and deep cryogenic treatment, the hardness behaviors produced different variation tendency and characteristics during aging According to obtained results and comparing with the characterization of carbides during aging, it showed that deep cryogenic treatment is an advantage for the formation and precipitation of alloy carbides during aging The results indicated that deep cryogenic treatment increases the diffusion driving force of atoms (especially the carbon atoms) which promotes the formation of fine carbides in the process of aging

The role of the microstructure on the influence of hydrogen on some advanced high-strength steels

Abstract: The role of microstructure was studied for dual-phase (DP), quenched and partitioned (Q&P), and twinning induced plasticity (TWIP) steels. The hydrogen influence changed the fracture mode at the ultimate tensile strength, there being no subcritical crack growth at a lower stress. The fractures initiated (i) in the hard martensite and/or at the interfaces of ferrite and martensite for DP steels, (ii) in the martensite and/or at the interfaces of retained austenite and martensite for Q&P steels, and (iii) at mechanical twins for TWIP steels. Tempering may improve the resistance to hydrogen of DP and Q&P steels.

Microstructure and mechanical properties of X65MoCrWV3-2 cold-work tool steel produced by selective laser melting

Abstract: In this study, martensitic cold-work tool steel X65MoCrWV3-2 was processed by selective laser melting (SLM) by varying the laser scanning parameters and baseplate preheating temperatures. Porosity as well as crack density of the SLM-densified steel were determined by quantitative image analysis. The resulting microstructure and the associated local mechanical properties were characterized, and the hardness-tempering behavior of the SLM-densified steel was compared to the behavior of the conventionally manufactured X65MoCrWV3-2 steel in the cast and hot-formed condition. Regardless of the preheating temperature, SLM-densified X65MoCrWV3-2 possesses a porosity of less than 0.5 vol.-%. The crack density was reduced significantly by means of a higher preheating temperature. The microstructure after SLM densification shows a fine, equiaxed cellular-dendritic subgrain structure, superimposed by lath- or needle-like martensite. The martensite morphology appeared to be finer at a lower preheating temperature, whereas the observed subgrain structure did not seem to be influenced by the preheating temperatures. Microhardness measurements indicated tempering effects in first solidified layers caused by the densification of subsequently deposited layers. Peak hardness after tempering of the SLM-densified steel was found to be higher compared to the maximum hardness in the X65MoCrWV3-2 steel in the cast condition.

In-situ quench and tempering for microstructure control and enhanced mechanical properties of laser cladded AISI 420 stainless steel powder on 300M steel substrates

Abstract: Laser cladding offers a very promising path to restore the geometry and structural integrity of ultra-high strength steel components that have suffered from fatigue cracking and wear damage. However, the overall tensile properties of the clad part often result in very poor ductility and low toughness due to untempered martensite formed in the clad layer and Heat-Affected Zone (HAZ). The fatigue properties are also adversely affected as a result of the poor ductility. To overcome this issue, this paper explores controlling the microstructure to achieve enhanced tensile properties. A laser idle time between each clad track was introduced to control the in-situ quench and tempering sequence. The microstructure, tensile, and wear properties of a geometrically repaired ultra-high strength martensitic 300M steel were evaluated. It was found that incorporating a long idle time between each laser clad track produced a mostly tempered martensitic structure in the clad layer resulting in ductility of 17.4% when compared to a deposit without in-situ quench and tempering control of 2.7%. Furthermore, the tensile and wear properties with the deposited layer with idle time were similar to that of the 300M steel baseline. The in-situ control of tempered martensite (through the longer idle time between clad tracks) is a critical process step in the laser cladding repair of aerospace steel components.

Microstructural evolution and deformation behavior of fiber laser welded QP980 steel joint

Abstract: Advanced high strength steels for the auto industry are in high demand regarding the development of new type of steels, reliable and economic welding techniques, etc. In this study, the third generation advanced high strength steels, cold-rolled quenching and partitioning steels (QP980), were butt-welded using fiber laser to evaluate the microstructure, mechanical properties and formability of joint. The fusion zone composed of lath martensite with high microhardness (499 HV). Martensite tempering and carbide precipitation were found at the sub-critical heat affected zone, resulting in a slight decrease of microhardness (~ 37 HV drop). The presence of joint had no effect on tensile strength and all welded samples failed at the base metal with 98.8% joint efficiency. The formability of welded blanks in Erichsen cupping test was dependent on the weld line position. The formability ratio was only 68.08% of base metal when weld line offset was 0 mm, and increased as the weld line moving away from the blank center because more deformation was transferred to base metal. Stretch-flange-formability of the joint in hole expanding test was inferior to that of base metal due to the high volume fraction of coarse martensite in fusion zone.

Influence of intercritical tempering temperature on impact toughness of a quenched and tempered medium-Mn steel: Intercritical tempering versus traditional tempering

Abstract: The influence of intercritical tempering temperature on impact toughness of quenched and tempered 0.05C-5.42Mn medium-Mn steel was studied and compared with traditional tempering. The experimental steel had high hardenability because of high Mn-content. Lath-like α'-martensite without retained austenite was obtained over a wide range of quenching rate of 0.5–30 °C/s, and the quenched steel showed high strength but low impact toughness. On intercritical tempering at 625 °C and 665 °C, the impact toughness was enhanced, as compared to traditional tempering at 570 °C. The reversed austenite enriched with Mn and C formed between the martensite laths was the underlying reason for the increased absorbed crack propagation energy, and the ductile-brittle transition temperature (DBTT) was reduced because of increased stability of reversed austenite. Compared to the steel tempered at 625 °C, the steel tempered at 665 °C contained more reversed austenite, but the reversed austenite was less stable because of reduced enrichment of Mn and C. The enrichment or depletion of Mn and C in austenite and martensite was thermodynamically studied by DICTRA. In striking contrast to the steels tempered between 625 and 665 °C, twinned martensite was formed in the steel tempered at high temperature of 700 °C, and the steel exhibited impact toughness lower than the quenched steel.

Characterisation of precipitation and carbide coarsening in low carbon low alloy Q&T steels during the early stages of tempering

Abstract: In order to ensure that appropriate tempering conditions are used to obtain desired strength and toughness for low carbon low alloy quench and tempered (QT furthermore, finer elliptical secondary Mo-V-rich carbides are observed after tempering for 4 h The coarsening of cementite contributing to the softening process in the three steels has been quantified with, most significantly, the inter- and intra-lath carbides coarsening independently Although fine secondary alloy carbides are observed after 4 h tempering, they do not result in any noticeable secondary hardening peak in the Base-Mo-V and Base-Cr-Mo-V-Si steels

In situ postweld heat treatment of transformation induced plasticity steel resistance spot welds

Abstract: Transformation-induced plasticity (TRIP) steel resistance spot welds are delicate to low-energy interfacial failure via crack propagation through martensitic fusion zone during cross-tension (CT) loading. This paper addresses the effect of three different types of in situ postweld heat treatment (PWHT) on the mechanical properties of TRIP steel resistance spot welds. Depending on the post weld second pulse current level, three different strengthening mechanisms were found including (i) martensite tempering with reduced hardness, (ii) refining of martensite packets with improved toughness and (iii) nugget re-melting/enlargement combined with possible reduction of grain boundary impurity segregation. All designed in situ PWHTs were enabled to promote pullout failure mode with improved load-bearing capacity and energy absorption capability during CT loading.