Analysis on the Operation Methods of Rail Welding and Postweld Heat Treatment in the Track Change Overhaul of Existing Railway Lines

Based on the analysis of operation methods of rail welding and postweld heat treatment in the track change overhaul of existing railway lines at home and abroad, this paper puts forward the existing problems in the operation methods of off-line welding and on-line welding in China, and puts forward the solutions to the existing problems. When this operation mode is used for on-line welding, it can reduce the quality risk in the heat treatment process of low-temperature locking joint; in case of broken track and urgent repair of broken track, this method can eliminate secondary shunting and facilitate the recovery of the line as soon as possible.

Influence of the heat transfer coefficient on the level of residual stress after heat treatment of the VVER-1000 reactor baffle

Background. Improvement of the methodology for the computational analysis of residual stresses in the structural elements of the reactor is an integral part of the work when extending the service life of NPP power units. Objective. Determine the value of residual technological stress arising in the baffle of a VVER-1000 reactor during welding and postweld heat treatment according to the austenitizing mode. To evaluate the effect of considering the dependence of the heat transfer coefficient on the temperature of the baffle surface at cooling in air during heat treatment. Methods. Numerical modeling of the stress-strain state of the baffle during welding and postweld heat treatment was carried out using the finite element method. Results. It was determined that in the process of heat treatment according to the austenitizing mode, the residual welding stress in the baffle are almost completely relaxed. Due to the high temperature gradient during rapid cooling in air after heating in the process of austenitization, new rather high residual stresses are formed in the zones of the baffle with the greatest metal thickness. Conclusions. Based on the results of the investigation, a high level of residual technological stress was determined, which should be considered when calculating the justification for extending the service life of the VVER-1000 reactor baffle.

Shielding Gas and Inclusion Content Effects on Impact Toughness and Tensile Properties of 410NiMo Steel Welds

The effect of shielding gas on the mechanical and microstructural characteristics of ER410NiMo martensitic stainless steel weldments was investigated. Three weldments with various inclusion contents were manufactured using different shielding gas compositions and welding processes: gas metal arc welding (GMAW) with 100% argon (Ar), GMAW 85% Ar/15% carbon dioxide (CO2), and flux cored arc welding (FCAW) 75% Ar/25% CO2. The inclusions in each weldment were characterized by means of scanning electron microscope observations and energy-dispersive spectroscopy analysis. The weldments underwent postweld heat treatment, after which the chemical composition and reformed austenite proportion were measured to account for microstructural effects. Hardness measurements, tensile tests, and impact toughness tests using the Charpy method were performed. The results showed that the Charpy V-notch (CVN) absorbed energy decreases with increasing inclusion content. The highest CVN absorbed energy, 195 J, was obtained for the GMAW 100% Ar weld, which had the lowest inclusion content. GMAW 85% Ar/15% CO2, with four times more inclusions than the former, had a CVN absorbed energy of 63 J. The current manufacturing process, FCAW 75% Ar/25% CO2, was found to have an inclusion content three times higher than the GMAW 100% Ar weld but a CVN absorbed energy of 66 J, which is close to the GMAW 85% Ar/15% CO2 weld. The results showed that using GMAW 100% Ar as a replacement to FCAW 75% Ar/25 % CO2 would lead to a three-fold improvement in terms of absorbed impact energy. The effect of inclusions on tensile properties, which was not clearly identified as several factors, in addition to inclusion content, affects the weld strength and elongation. Overall, the yield and ultimate tensile strengths differed slightly: 724 and 918 MPa for GMAW 100% Ar, 746 and 927 MPa for GMAW 85% Ar/15% CO2, and 711 and 864 MPa for FCAW 75% Ar/25% CO2, respectively.

Filler Metal 16-8-2 for Structural Welds on 304H and 347H Stainless Steels for High-Temperature Service

The use of Type 16-8-2 filler metal was examined for application in structural welds on 304H and 347H stainless steels for high-temperature service applications and compared to welds with matching filler metals 308H and 347, respectively. Microstructural stability during elevated temperature expo-sure, weld metal impact properties, and susceptibility to stress-relief cracking were examined. It was found that the lean composition and low ferrite (~ 2 Ferrite Number [FN]) in 16-8-2 weld metal provide high resistance to intermetallic phase formation. No hot cracking was observed despite the low ferrite level. The 16-8-2 weld metals displayed superior toughness as compared to the matching filler metal welds, especially after longer elevated-temperature exposure. Experimental evidence for some martensite transformation in aged 16-8-2 weld metal upon cooling to ambient temperature was presented and explained an increase in magnetic response (as FN) after postweld heat treatment at 1300˚F (705˚C). None of the tested weld metals failed by stress-relief cracking mechanisms under the applied test conditions. The 16-8-2 filler metal welds exhibited significantly lower levels of stress relief during high-temperature exposure and significantly high-er tensile strength after high-temperature hold as compared to the matching filler metal welds.

Effects of Postweld Heat Treatment on the Phase Transformation and Mechanical Behavior of NiTi Ultrasonic Spot Welded Joints With Al Interlayer

The influence of postweld heat treatments (PWHTs) on the phase transformation characteristics, microstructural evolution, and mechanical properties of ultrasonic spot welded NiTi alloy with Al interlayer was investigated. At room temperature, the as-welded joints were fully austenitic, while the microstructure of welded joint after PWHT at 450 °C consisted of a mixture of martensite and austenite. The transformation temperatures were found to increase after the PWHT. Transmission electron microscope analysis shows that Al3Ti intermetallic compounds were formed on the Al side of the Al/NiTi interface after the PWHT at 450 °C for 90 min. Furthermore, finely dispersed nanoscale Ni4Ti3 precipitates were observed in the joint. These nanoscale precipitates resulted in an increase in the transformation temperatures and improved the mechanical properties of the PWHT material. During lap shear test, the as-welded samples failed in a brittle manner in the NiTi alloy, while the PWHT samples failed in the Al interlayer and exhibited ductile-like fracture characteristics.

PWHT Exemptions for Carbon Steel in B31.3: An EPC Contractor’s Perspective

In the 2014 edition of the ASME B31.3 Code [1], thick, P-No. 1 carbon steel pipe became exempt from Postweld Heat Treatment (PWHT) under the condition of preheating at 95°C (200°F) and multi-pass welding. The decision whether to apply PWHT or not is left to the designer but no further guideline is provided. The impression is that the need for PWHT is only corrosion/service related, and not beneficial or necessary for the integrity of the piping. Several publications [2][3][4] have addressed these changes and highlighted that this might lead to potentially unsafe situations. This paper will critically review the arguments used for the justification of the PWHT exemption for carbon steel and show that many arguments are invalid or incomplete. It will discuss the implications for the performance of materials and predict possible failure scenarios. It will then provide estimates of typical PWHT cost eliminated by the current rules. It will provide an EPC contractor’s perspective on the current ASME B31 rules with practical approaches that may be taken to mitigate the risks. Finally, recommendations to the ASME B31 committees involved in PWHT exemption will be provided.

Tempering Response in Type 410 Stainless Steel Welds for Petrochemical Application

Type 410 martensitic stainless steel is typically used in highly corrosive environments within petrochemical installations due to its resistance to halide stress corrosion cracking, hardenability, and low cost compared to austenitic stainless steel. However, the industry has experienced difficulties in meeting the ASME toughness, and NACE hardness requirements for wet sour services of Type 410 steel welds. Recent studies have shown that these problems are related to the wide compositional ranges of Type 410 base metals and welding consumables, leading to exceeding the A1 temperature during postweld heat treatment (PWHT) and formation of fresh martensite, and to retention of significant amount of delta ferrite in the final weld metal and heat affected zone microstructures. These studies have identified two Type 410 optimized weld metal compositions that met the specified hardness and toughness requirements. The objective of this work was to quantify the tempering response in one of the optimized welding consumables and in two Type 410 base metals. Samples of these materials were subjected to a series of PWHTs at temperatures corresponding to the lower and upper limits of the ASME code recommended temperature range (760 C and 800 °C) and at 10 °C below the A1 temperature of each material. The PWHT durations were 5 and 30 minutes, and 1, 2, and 4 hours. The hardness values related to all PWHTs performed below the corresponding A1 temperatures were used to generate Holloman–Jaffe type equations for all tested materials. As expected, the PWHTs performed above the A1 temperatures resulted in the formation of fresh martensite.