Papers by Keyword: Temper Embrittlement

Abstract: The forming at elevated temperatures for Advanced High Strength Steels (AHSS) opens up a new technology. The phase changes during warm deformation are the key to understanding the warm forming process. The desired microstructure and mechanical properties before and after warm forming have to be known in order to find optimal conditions for achieving good sheet formability and preferred material properties in service. In this work, the TRIP690 and DP780 steels are investigated under punch stretch test conditions in order to evaluate the temperature influence on neck formation and fracture occurrence at ambient and elevated temperatures 200oC, 400oC. Contact heat treatment was used for heating up the circular specimens. It was found that formability of the investigated steels was drastically reduced at a temperature of 400oC and brittle fracture occurred because of temper embrittlement. It is recommended to avoid steel tempering at this critical temperature.

Abstract: The band saw steel must present a good balance between strength, toughness and materials elasticity in the weld joint, respectively in the HAZ. Those areas are affected by the heat welding cycle that is changing the material microstructure and consequently the overall mechanical properties of the saw blade. Minor changes in the chemical compositions of the saw blade material, in welding parameters or in the post-welding treatment can cause semnificative changes in the saw blade durability and performance. A SMEs from Cluj-Napoca had encountered a problem after the welding operation of the 51CrV4 QT steels, bought from two different European manufacturers. It was observed a different welding behavior for the same steel bought from one of the steel manufacturer compared to the other one. The SMEs requested a study in order to find out what went wrong during welding operation, because the bending test has shown an insufficient toughness in the welded joint. As result, we have conducted a research in the quest of the causes for the embrittlement phenomena observed in the welded joint. Chemical analysis, mechanical testing, SEM and EDX and XRD analysis have been used to investigate the material properties in the parent material, weld and HAZ.

Abstract: This work studied the interaction between carbide precipitation and impurity segregation under temper embrittlement (TE) conditions in a coarse-grained heat-affected zone (CGHAZ) in Q690 steel, a low-alloy high-strength structural steel used in the hydraulic support in the fully-mechanized mining face. From the perspective of carbide precipitation, through thermodynamics calculation and analysis, it was found that the existence of cementite at the grain boundaries was preceded by impurities segregation (primarily phosphorus). The precedent phosphorus segregation thus enhances the nucleation rate of cementite at the grain boundaries by lowering the ferrite/cementite interfacial energy. Both carbide precipitation and impurity segregation at the grain boundaries reached a maximum as a result of their mutual role in the temperature range of TE.

Abstract: The effect of post-weld heat treatment (PWHT) on carbide precipitation and impact properties of coarse-grained heat-affected zone (CGHAZ) of Q690 Steel was studied in this paper. Carbide particles precipitated primarily at prior austenite grain boundaries and martensitic lath boundaries. When the PWHT temperature is 520–570 °C, temper embrittlement occurs. This temperature range is also where the number of carbide particles per unit area at grain boundaries reaches its maximum. The high number of particles per unit area increases the rate of crack initiation at grain boundaries under rapid loading; linking of microcracks along grain boundaries which are already weakened by impurity segregation results in TE and intergranular fracture.

Abstract: We found that welding rod of the same trademark but different batch has distinct different tendency of brittleness when they are experienced step cooling embrittling treatment process.A series of 2.25Cr-1Mo-0.25V steel welds were prepared using the manual arc. The notch toughness properties were assessed in the PWHT condition and after a temper embrittlement step-cooling treatment. the J and X factors of the weld should be more rigorous than the base steel. low Mn and Si content weld metal is less susceptible to temper embrittlement. The temper embrittlement was evaluated as the shift of 54 Joule transition temperature (vTr54) by the Charpy v-notch impact tests. In the post-weld heat-treated condition (PWHT) the notch toughness requirement for weld metal is 54 J at-40°C .

Abstract: Martensitic grade stainless steels are being extensively used in aerospace, defence and nuclear sectors for structural applications. Specialized applications require close control in chemistry and heat treatment parameters. Control of carbon in AISI-420, alloying content has typical service advantages. To study the effect of tempering temperature with carbon content on mechanical properties, various heat treatment cycles were devised, avoiding the regime of temper embrittlement. This paper presents the tempering response of medium carbon stainless steel AISI-420 grade with respect to change in carbon content from 0.2% to 0.3%. It is observed that, by varying the tempering temperature, the grade can be tailored to obtain wide range of mechanical properties. With increasing carbon content, martensite is found to be changing in morphology from lath to lath & plate (mixed) and the amount of retained austenite also increases. An attempt has been also made to arrive at a structure-property correlation in this grade of stainless steel.

Abstract: Baysesian Neural Network approach for predicting the temper embrittlement of steam turbine rotor in service was proposed. The FATT50 (the fracture appearance transition temperature) of the rotors was predicted as a function of ratio of the two peak current densities (Ipr / Ip ) tested by electrochemical potentiodynamic reaction method, temperature of electrolyte, J-factor and grain size ( N ). A database was obtained from the test of electrochemical potentiodynamic reaction and Charpy impact. The Bayesian neural network technique was used for modeling of temper embrittlement. The neural network shows a more precise prediction of temper embrittlement of rotor steels than the prediction using multiple linear regression. The training error and verifying error is with the scatter of ±20°C. The results show that, for the temper embrittlement of rotor steels prediction, the prediction model based on Bayesian neural network is feasible and effective.

Abstract: When failure occurs in material, it is often occurs by fracture along some grain boundaries and often by the micro-segregation of embrittling impurity to the grain boundaries. In the present work, the non-equilibrium grain-boundary segregation (NGS) kinetics of phosphorus and the temper embrittlement at the same solution treatment and different isothermal holding temperature in steel 2.25Cr1Mo are studied. The NGS kinetics curves of phosphorus at the same solution temperature （1050 oC ）and different isothermal holding temperature (540 oC and 600 oC) are given. Experimental results provide a direct evidence of NGS kinetic model and show that the grain boundary segregation concentrations of phosphorus for specimen isothermal holding at 540 oC are higher than those at 600 oC. The peak values of AES patterns of solute atoms for specimen isothermal holding at 540 oC are also higher than those at 600 oC. It is therefore concluded that the lower the isothermal holding temperature, the higher the segregation concentration of phosphorus at the grain-boundaries, and also the higher the degree of embrittlement.

Abstract: Temper embrittlement and fracture control method based on both non-equilibrium grain-boundary segregation (NGS) theory and grain refinement technique are studied in this paper. Grain refinement technique by deformation induced phase transformation in low-alloy steels, 12Gr1MoV and 2.25Gr1MoNb, is investigated. A single-pass hot rolling process by using a Gleeble-1500 system is performed. Experimental results show that steel strength and toughness may be controlled and improved by grain refinement, and that the grain sizes were affected by the deforming temperature, strain reduction, and strain rate. According to the NGS theory, a control method of brittle fracture along grain-boundary is proposed so that some catastrophically brittle fracture failure may be averted. Grain refinement may decrease both the concentration of phosphorus at grain boundaries and the critical time. With the grain refinement technique, the temper embrittlement of steel may be improved, and the critical time may be shortened. The cost of heat treatment for fracture control will therefore be reduced.

Abstract: The non-equilibrium grain-boundary segregation (NGS) isotherms and its kinetics serve to provide a more complete understanding of inter-granular segregation behavior in relation to mechanical properties, not only for the engineering steels but also for a wide range of structural alloys. The NGS of phosphorus and temper embrittlement dynamics on the same heat treatment condition at the same isothermal holding time in two Cr-Mo steels, 12Cr1MoV and 2.25Cr1Mo, was experimentally studied. The fracture behaviour was also observed by tensile tests in situ in a scanning electron microscope (SEM). Results show that both the concentration of phosphorus atoms in grain boundaries and the degree of embrittlement reaches a maximum at the critical time. It can be satisfactorily elucidated by the temper embrittlement mechanism of NGS caused by cooling from solution temperature to isothermal holding temperature.