Improving 410NiMo weld metal toughness by PWHT

Highlights of the present study is the importance of choosing suitable temperatures for two stage PWHT to achieve desirable toughness in the weld metals produced by ER 410NiMo filler wire. Weld pads prepared using this filler wire was used for extensive metallurgical characterization of the weld metal. Results indicate by choosing appropriate temperatures for the PWHT, it is possible to obtain toughness in the weld metal which is comparable to the toughness reported for the base metal of similar composition. Good toughness of the weld metal is attributed to the presence of retained austenite in the weld metal. Two stage PWHT that gave excellent toughness for the weld metal was employed for repair of cracked shrouds of a steam turbine in a nuclear power plant. The metallurgical characterization of the mock up weld pad prepared prior to actual repair confirmed that microstructure and hardness of the weld metal are similar to those obtained during the welding procedure development.     

Mechanical properties of 422 stainless steel repaired via wire feed laser welding

Abstract

Laser welding with filler wire additions could be used in restoration of components that are of high cost or sometimes difficult to procure, such as steam turbine blades in fossil fuel power plants. In the present work, machined V groove specimens were employed to simulate laser repair of Carpenter 636 stainless steel (SS), which has a similar composition to a blade material, type 422 SS. Before repair welding, a heat treatment procedure including solution and temper treatments of the specimens was carried out according to the mechanical and microstructural analyses of a used blade after 20 years service at about 540° C. Tensile, impact, and fatigue crack growth tests of weld repairs using 410 SS filler wire were conducted. The weld repairs exhibited an impact toughness similar to that of the base metal and a lower fatigue crack growth rate than the base metal. However, the lower hardness associated with 410 SS filler metal led to tensile fracture in the weld metal of repaired specimens. Accordingly, the use of 410 SS filler metal for repair welding type 422 SS components should be limited to regions under low stress.     

Effect of minor change in composition on toughness of weldmetal for repair of turbine blades made of martensitic stainless steel

Abstract

Repair welding procedure for cracked turbine blades, made of 13Cr–2˙6Ni–1˙1Mo martensitic stainless steel, has been developed using gas tungsten arc welding process and a twin wire filler metal. The twin wire consists of a 1˙5 mm diameter ER 16-8-2 and a 2˙0 mm diameter ER 410 filler wires tack welded along the length of the two filler wires. A two stage post-weld heat treatment at 675°C for 2 h and 615°C for 4 h, such that the first heat treatments is above the Ac ₁ temperatures of the weld metal and the second is just below its Ac ₁ temperature; has been found to be suitable for obtaining good mechanical properties for the weldment. The weldment has a good combination of transverse weldment strength and weldmetal toughness, with its room temperature yield strength and Charpy V notch impact toughness being similar to that of the turbine blade material.     

Effects of ac arc ultrasonic on plasma arc in situ welding of SiCp/6061Al MMCs

Abstract

The isolation coupling device is designed based on the principle of high pass filter circuit, which can achieve isolation and coupling between ac welding power and ultrasonic frequency excitation source. In addition, ac arc ultrasonic vibration is obtained successfully. The plasma arc in situ welding of SiCp/6061Al was performed using two different processes. The first process is that the plasma gas is argon nitrogen mixture and the filler material is flux cored wire (Al–5Ti–5Si); the other process is that the plasma gas is argon and the filler material is flux cored wire (Al–15Ti–3Si). The two joints are respectively enhanced by nitride and Al₃Ti, which were obtained in these processes. After arc ultrasonic vibrations were imposed on the two welding processes, it was revealed that arc ultrasonic improved the distribution of the new nitride phases and optimised the morphology of Al₃Ti to improve the mechanical properties of joints that reached 246 and 263 MPa. Fracture analysis showed that the improvement of the mechanical properties resulting from Al₃Ti is better than that from nitride under the effect of arc ultrasonic.     

Automatic TIG root pass welding,using a non-current-carrying filler wire in the vertical position

Abstract

The results of root pass welding trials using automatic TIG in the vertical position, both in the upward and downward directions is described. The welding was carried out introducing a non-current-carrying filler wire and using a modulated welding current. The filler wire, fed synchronously with the weld current modulation, allowed much better control of the weld pool even in this constrained position.     

Study on ac-PMIG welding of AZ31B magnesium alloy

Abstract

Welding of AZ31B magnesium alloy is carried out using alternating current pulsed metal inert gas (ac-PMIG) welding with 1·6 mm diameter of filler wire. Typical current waveform is used to make sure arc given an accurate energy input into filler wire. The arc characteristics, metal transfer forms, microstructure and mechanical property of ac-PMIG welding of AZ31B magnesium are investigated. The results show that a stable welding procedure and continuous joints can be obtained easily under a wide range of welding parameters. The most important factors for ac-PMIG welding are negative electrode (EN) ratio and pulse rework current, which give an accurate energy input into filler wire. The grain in fusion zone is much finer and more uniform, and grain size does not grow significantly in the heat affected zone compared with base metal. The average ultimate tensile strength of weld beads is 97·2% of base metal.     

Welding current prediction in GMAW and UGMAW processes using response surface methodology

Abstract

Among all process variables in gas metal arc welding (GMAW) process, welding current is the most influential variable affecting heat input and weld quality. Its dependence on other process variables in GMAW and universal gas metal arc welding (UGMAW) processes (which makes use of a specially designed torch to preheat the filler wire independently, before its emergence from the torch) has been investigated using four factor five level central composite rotatable design to develop relationship for predicting welding current, which enables to quantify the direct and interactive effects of four numeric factors, namely wire feedrate, open circuit voltage, welding speed and electrode stickout and one categorical factor preheat current. Mathematical models developed show that welding current increased linearly with increaseing wire feedrate and open circuit voltage, whereas it decreased with increasing electrode stickout and preheat current. Numerical optimisation was carried out, and the optimal solutions generated indicate that under the same input conditions higher deposition rates are achievable in UGMAW process.     

Prediction of dilution in GMA and UGMA stainless steel single layer cladding using response surface methodology

Abstract

This paper describes the use of a variant of GMAW process named as UGMAW (universal gas metal arc welding) process for single layer stainless steel cladding, which makes use of a specially designed torch to preheat the filler wire, using an auxiliary welding power source, before its emergence from the torch. The experimental work undertaken was that of single layer cladding of 12 mm thick low carbon steel with the austenitic stainless steel 316L solid filler wire of 1·14 mm diameter. Dependence of dilution was investigated using four factor five level central composite rotatable design to develop relationship for predicting dilution, which enables to quantify the direct and interactive effects of four numerical factors, namely, wire feed rate, open circuit voltage (OCV), welding speed, electrode stickout and one categorical factor, preheat current. External preheating of the filler wire in UGMAW process resulted in greater contribution of arc energy by resistive heating owing to which significant drop in the main welding current and hence low dilution values were observed. Numerical optimisation was carried out and the optimal solutions generated indicate that for same levels of dilution, higher deposition rates are achievable in UGMAW process, thus making it a good choice for low cost surfacing applications.     

Fracture toughness of CA6NM alloy,quenched and tempered,and of its welded joint without PWHT

CA6NM quenched and tempered steel is used in hydraulic turbine rotors, pumps and compressors. The objective of this research is to determine the fracture toughness of tempered and quenched CA6NM alloy, and of its welded joints without post-welded heat treatment (PWHT). To this end, compact tension (CT) test pieces are milled from pieces of CA6NM steel for evaluation of the toughness of the alloy used in a hydraulic turbine. Due to the elasto-plastic condition of the material, the test pieces are tested by means of the J integral concept, setting out the resistance curve J–R and the crack initiation J _IC. In welded joints produced from ingots, without PWHT, the fragility they show does not allow the J–R curve for the CT test pieces to be drawn up, and the toughness is characterized by means of the K _IC concept. The welding procedure looks at the probable conditions for repair of cavitation wear to the turbine, where PWHT cannot be carried out. The results confirmed the higher toughness for the CA6NM steel, with values approximately three times higher than those obtained in the welded joints without PWHT. In terms of the fracture, the CA6NM steel shows ductile behaviour while the welded joint without PWHT shows fragile behaviour.     

Evolution of residual stresses in dissimilar steel surfacing layers during process of post-weld heat treatment

Abstract

Residual stress relaxation during post-weld heat treatment (PWHT) is a thermodynamic process, which is affected not only by the heat treatment process, but also by the welding residual stress. In this study, the residual stresses in as welded and heat treated surfacing metal were measured using blind hole and X-ray method. The results reveal that the welding residual stresses are compressive at the surface of the weld and tensile at inner weld. However, after PWHT, the residual stresses at surface and inner weld change to the opposite state. Finite element simulations show that the differences of expansion coefficients between base metal and filler material are the main factor to the changes of stress state. The experimental results verify that the expansion coefficients of base metal and filler materials have been changed greatly after long soaking at high temperature.     

Analysis of thermal cycles in resistance flash welding of steels

A comparative study of the clad electrode and tubular wire processes was carried out, looking at the mechanical and micro-structural properties of weld metals of high-resistance steel, in the ‘as-welded’ and ‘stress-relief heat treatment’ (PWHT) conditions. The results show that the procedures adopted for welding with tubular wire and clad electrodes allow satisfactory levels of mechanical resistance to be obtained, with the exception of the value of percentage lengthening of the tubular wire, in the as-welded condition. The impact tests show that both the weld metals showed satisfactory impact resistance, in both the as-welded and PWHT conditions, observing that, for the tubular wire, the impact resistance is lower for the clad electrode in both conditions, lying close to the limit applied for the acceptance criterion of 50 J at 0°C in the PWHT condition. It was confirmed that the productivity achieved by the tubular wire process was approximately twice as high as that for the clad electrode process. As a result, the inherent advantage of the tubular wire process must be complemented with a consideration of all the mechanical properties obtained, orienting suitable selection of the welding process, in particular, for application in equipment that operates in fatigue conditions.     

Recent developments in repair welding technologies in Japan

Abstract

Developments in some difficult repair welding technologies in Japan during the past decade are reviewed. The topics covered include the repair welding of bridges in service, the temper bead method which makes it possible to omit post-weld heat treatment (PWHT) of repaired pressure vessels, the seal welding of a reactor vessel in which stress corrosion cracks were detected, low heat input repair welding of neutron irradiated stainless steel and nickel based alloys, the prevention of solidification cracking in repair welding of aged heat resistant cast steels, the development of welding materials for the mending of single crystal nickel based superalloy turbine blades, underwater repair welding of nuclear reactors, the reduction of residual stresses in repair welding, and an ultrasonic testing method for nickel based weld metals. The local PWHT of creep resistant ferritic steel tubes is also reviewed.     

Effect of variant transformations in fusion zones of gas metal arc welds

Abstract

The macro- and microtextures of gas metal arc welds fabricated using both conventional ferritic and low transformation temperature (LTT) filler metals were examined. Both welds were largely composed of acicular ferrite. The weld textures were found to be distinct, which is observed to be due to variant selection during the austenite to ferrite transformation. In situ electron backscatter diffraction performed during thermal cycling of the LTT fusion zone into the austenite phase field confirmed that these transformations satisfy the expected crystallographic relationships. Implications for welding consumable design and finite element process modelling are drawn.     

Use of acoustic emission to detect reheat cracks during post-weld heat treatment

Abstract

The phenomenon of stress relief cracking or reheat cracking can occur in the heat affected zone (HAZ) of welded joints during post-weld heat treatment (PWHT). The coarse grained region, which results from heating at elevated temperatures in the austenitic region, is the most susceptible to this problem. The present research programme was aimed at determining, by means of an acoustic emission measurement system (AEMS), the instant of propagation of reheat cracks during PWHT. The AEMS was fixed to a modified implant test device that allowed the loading of a specimen subjected to thermal cycles similar to those typically experienced by welded components in stress relief treatments. Tests were carried out with this setup using different load levels for the same thermal cycle and welding condition. Welding was done by means of the metal active gas process. The study was developed using a high strength low alloy (HSLA) steel commercially produced in Brazil. Test results have shown that the AEMS is adequate for monitoring the formation and growth of reheat cracking during PWHT.     

Effect of filler wire on the joint properties of AZ31 magnesium alloys using CO2 laser welding

Laser welding with filler wire of AZ31 magnesium alloys is investigated using a CO2 laser experimental system. The effect of three different filler wires on the joint properties is researched. The results show that the weld appearance can be effectively improved when using laser welding with filler wire. The microhardness and tensile strength of joints are almost the same us those of the base metal when ER AZ31 or ER AZ61 wire is adopted. However, when the filler wire of ER 5356 aluminum alloy is used, the mechanical properties of flints become worse. For ER AZ31 and ER AZ61 filler wires, the microstructure of weld zone slws small dendrite grains. In comparison, for ER 5356 filler wire, the weld shows a structure of snowy dendrites and many intermetallic compounds and eutectic phases distribute in the dendrites. These intermetallic constituents with low melting point increase the tendency of hot crack and result in fiagile joint properties. Therefore, ER AZ31 and ER AZ61 wire are more suitable filler material than ER 5356 for CO2 laser welding of AZ31 magnesium alloys.     

Laser welding of Mg alloy MgAl3Zn1 (AZ31) to Al alloy AlMg3 (AA5754) using ZnAl filler material

Abstract

Welding of magnesium with aluminium alloys is challenging due to the formation of brittle intermetallic phases. In this study laser welding of an Mg alloy with an Al alloy, using a Nd∶YAG laser with a two focus optics and ZnAl filler material, was investigated. Mechanical tests, as well as microstructure investigations by light optical microscopy, scanning electron microscopy and energy dispersive X-ray microanalysis were carried out in order to characterise the hybrid joint. The strength of the joints is significantly affected by the aluminium content in the filler wire. Strengths of the joint were comparable to those obtained by other joining techniques such as diffusion or adhesive bonding using welding speeds of 1·75 m min^?1.     

In situ analysis of the strain evolution during welding using low transformation temperature filler materials

ABSTRACT

Compared to conventional welding consumables using low transformation temperature (LTT) filler materials is an innovative method to mitigate tensile residual stresses due to delayed martensite transformation of the weld. For the effective usage of LTT filler materials, a deeper understanding of the complex processes that lead to the final residual stress state during multi-pass welding is necessary. Transformation kinetics and the strain evolution of multi-pass welds during welding were investigated in situ at the beamline HEMS@PETRAIII, Germany. Compared to conventional welds, the total strain was reduced and compression strain was achieved when using LTT filler materials. For an optimal use of the LTT effect in the root of multi-pass welds, the alloying concept must be adapted taking care of dilution.     

Search for filler metal for welding of ferrous alloys to titanium

Abstract

The use of a filler metal to facilitate the gas tungsten arc (GTA) welding of ferrous alloys to titanium alloys has been investigated. Semi-empirical rules have been applied to identify alloying elements that would resolve the important problems of brittle intermetallic formation and weld cracking. Vanadium was found appropriate. The GTA welds between a low carbon steel and Ti–15V–3Cr–3Sn–3Al made with a vanadium filler wire resisted cracking better than comparable autogenous welds. However, the presence of a hard, brittle eutectic microconstituent along the ferrous side of the fusion boundary drastically limited the gain in weldability. As anticipated, analysis of GTA welds produced with vanadium filler wire suggested the presence of a ternary (Fe,Ti,V) single phase. Although cracking was reduced with vanadium, the practical benefit of a vanadium filler wire for GTA welding is small because the weld metals remain very hard and brittle.     

Microstructure and bend strength of WC–Co and steel joints

Abstract

Tungsten inert gas arc welding of WC–30 wt-%Co cemented carbide and steel was carried out using a variety of filler alloys. Precipitation of Fe₃W₃C occurred at the joint interface and this reduced the bend strength and toughness of the welded joint. The problem could be alleviated by adjusting the carbon concentration of the filler metal.     

Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding

Abstract

The effect of welding parameters on the distribution of wire feeding elements has been investigated during CO₂ laser and pulsed gas metal arc hybrid welding process. The molten metal flow on the pool surface and inside of the samples was observed by a high speed video camera and an in situ X-ray transmission imaging system respectively. The results indicate that the fluid flow towards the inside of keyhole, namely inward flow, improves the homogeneity of weld metal. The distribution of alloying elements is more homogeneous in leading laser compared with leading arc, since both of the drag force of the plasma jet and momentum of droplet promote the inward flow in leading laser. Almost homogeneous distribution of alloying elements can be attained if the oxygen content in the shielding gas is more than 2%, since the Marangoni flow direction changes from outward to inward with increasing the oxygen content.