Reheat Cracking Research Articles

Analysis and Repair of Cracks in Welding Tee of Main Steam Pipelines in Thermal Power Units

Abstract During the maintenance process of a certain thermal power unit, multiple intermittent cracks were found in the Y-type welding tee weld of the main steam pipeline. This paper expounds the detection situation and treatment process of the welding tee weld, and analyzes the material, operating condition, structural characteristics and stress state of the test parts. The results show that the cracks are reheat crack, which is greatly related to the material. In addition, the structure of the welding tee is not reasonable, which increases the stress of the weld and accelerates the generation and expansion of the cracks. In this paper, the optimization suggestions are put forward for the welding repair and heat treatment process of the main steam pipeline, and the defects are successfully treated, which provides a reference for the detection and repair of the weld defects of similar and thick-wall pipe fittings.

Effect of metastable precipitate phase transformation on reheat cracking in SS347H welding

Reheat cracking occurred in the welded heat affected zone (HAZ) of the SS347H tube during high temperature applications of approximately 600 °C. The reheat cracks occur when the inside of the grain is strengthened, causing grain boundary embrittlement. In this study, the intra-granular strengthening phase transformation of the HAZ causing the reheat crack was analyzed using TEM.The hardness of SS 347H HAZ with cracks is measured to be 221HV on average, which is about 25HV higher than 196HV of HAZ without cracks. This intra-granular strengthening is due to the formation of intra-granular precipitate phases. The precipitated phase is ultimately NbC, but undergoes an intermediate phase transformation due to diffusion of Nb and C components. The phase transformation consists of the following steps: “(i) SA (solution annealing), (ii) Zone A, (iii) Zone B, and (iv) NbC precipitation.”This is a process in which Nb/C components diffuse and segregate at the reheating temperature from a solid solution state of Nb/C components, and NbC with a size of several tens of nanometers is precipitated. As metastable phases, Zones A and B exist in the intermediate process of precipitation, which is the strengthening stage where intra-granular hardness is maximized. Zone A and B in the pre-precipitation stage means that a new lattice plane is formed as the segregation of component elements forms coherency with the matrix. The relationship between the base and new lattice planes is confirmed by Double Diffraction in the TEM diffraction diagram.The root cause of reheat cracking in SS 347H HAZ is believed to be the intra-granular strengthening phase caused by zone A and zone B as a metastable phase transformation in an intermediate stage before NbC precipitation. In order to suppress reheat cracking, it is desirable to precipitate stable NbC from the metastable phases of Zones A and Zone B through stabilization heat treatment.

Influence of Martensite–Austenite Constituents and Precipitates on Reheat Cracking Susceptibility of Coarse-Grained Heat-Affected Zone in T23 and Modified T23 Steels

Influence of Martensite–Austenite Constituents and Precipitates on Reheat Cracking Susceptibility of Coarse-Grained Heat-Affected Zone in T23 and Modified T23 Steels

Study on the reheat crack sensitivity in simulated coarse-grained heat-affected zone of 7CrMoVTiB10-10 steel

Study on the reheat crack sensitivity in simulated coarse-grained heat-affected zone of 7CrMoVTiB10-10 steel

Interface Characterization of Bimetallic Ti-6Al-4V/Ti2AlNb Structures Prepared by Selective Laser Melting.

Additive Manufacturing (AM) of multimaterial components is a promising way of fabricating parts with improved functional properties. It allows for the combination of materials with different properties into a single component. The Ti2AlNb-based intermetallic alloy provides high temperature strength, while the Ti-6Al-4V (Ti64) alloy has good fracture toughness, ductility, and a relatively low cost. A combination of these alloys into a single component can be used to produce advanced multimaterial parts. In this work, Ti2AlNb/Ti-6Al-4V bimetallic structures were fabricated from pre-alloyed powders using the Selective Laser Melting (SLM) process. The effects of high-temperature substrate preheating, post-processing by annealing, and hot isostatic pressing on defect formation, the microstructural evolution of the interface area, and the mechanical properties of the bimetallic samples were investigated. High-temperature substrate preheating during the SLM process was necessary to prevent reheat cracking of the Ti2AlNb part, while annealing and hot isostatic pressing post-processing improved the chemical and microstructural homogeneity of the transition zone and enhanced the tensile properties of the bimetallic structure.

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Study on Susceptibility and Mechanism of Reheat Cracking in Welded Joint of Type 347 Austenitic Stainless Steel

The reheat cracking susceptibility of welded joints has been evaluated by 900°C high-temperature constant load test and 750°C–900°C high-temperature slow strain rate tensile (SSRT) test on welded joints of TP347 steel, and the influencing factors and mechanism of reheat cracking in welded joints have been further investigated by microcosmic characterization techniques of SEM and EDS, etc. The result shows the critical fracture stress of TP347 steel welded joints at 900°C is 24 MPa, which is much lower than 80% of the high-temperature yield strength of base material, indicating that the reheat crack of the welded joint is highly sensitive at this temperature; the average reduction of area (RoA) of welded joints obtained by 750°C–900°C SSRT test at each temperature is below 20%, which means all welded joints have a reheat cracking susceptibility in this temperature range, and the reheat cracking susceptibility increases with the rising of temperature. Based on study and analysis using microcosmic methods of optical microscope, scanning electron microscope, and energy spectrum analysis, it is found that welded joints of TP347 steel have a high reheat cracking susceptibility because the high temperature accelerates the diffusion of Cr, Nb, and impurity elements to the grain boundary, and a large quantity of carbides and low-melting-point eutectics are collected and precipitated at the grain boundary to reduce the plasticity of the grain boundary. Since there is a high welding residual stress in welded joints, cavities are formed under the action of the stress at a high temperature, and the merging of cavities causes the formation of cracks which extend along the grain boundary.

Effect of Precipitates and Cold Working on the Reheat Cracking of STS 347 Austenitic Stainless Steel

The aim of this study was to investigate the effect of precipitates and cold working on the reheat cracking of STS 347 austenitic stainless steel. Three different heat treatments to form NbC and M23C6 precipitates were applied to the STS 347 austenitic stainless steel and 10% pre-strain was additionally introduced to simulate cold working. The NbC and M23C6 precipitates were examined by SEM and XRD analysis, and creep rupture test was conducted under various temperature and stress conditions. The specimen having intergranular NbC precipitate prior to cold working had enhanced intergranular creep resistance and the tendency to reheat cracking decreased at temperature conditions between 650 oC and 750 oC, compared to the specimens processed by solution treatment or M23C6 formation treatment. This means that for STS 347 that has not been subjected to stabilization treatment, the cold working strain limits required by ASME’s boiler and pressure vessel code should be lowered, or if stress relief treatment is impractical, the stabilization heat treatment should be performed prior to cold working. Although the precipitation of M23C6 is not the major cause of reheat cracking, it should still be considered as a factor that increase the sensitivity of reheat cracking because M23C6 forms mostly at the grain boundary and grows rapidly, adversely affects creep ductility.

Effects of Stabilizing Heat Treatment on Properties of Simulated Microstructures in the Heat-Affected Zone of 347H Stainless Steel

In this paper, two kinds of heat affected zone (HAZ) simulation structures of 347H stainless steel, which are coarse grain zone (CGZ) and unmixed zone (UZ), were prepared by thermal simulator. The material properties of toughness, reheat crack susceptibility and intergranular corrosion susceptibility of the two kinds of HAZ simulation structures were studied by impact test, high temperature tensile test, electrochemical potentiodynamic reactivation (EPR) test and micro morphology test. The result shows that CGHAZ had better toughness. But after the stabilizing heat treatment, it was weakened while that of the UZ was enhanced. The reheat crack susceptibility of the CGZ and UZ both increases after stabilization heat treatment, and the tendency of the UZ are more obvious. Stabilizing heat treatment has a certain effect on the prevention of sensitization process, which can improve the intergranular corrosion resistance of the material. Stabilizing heat treatment is double-edged to 347H HAZ, and it needs combined with the specific situation to used.

Investigation of reheat cracking susceptibility in CGHAZ of modified T23 steel

T23 steel (2.25Cr–1Mo–1.6W–0.24V–0.05Nb, in wt.%) has been widely used as water walls in ultra-supercritical (USC) power plants. However, high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants. It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition. Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone (CGHAZ) in T23 and modified T23 steels via thermal simulation of welding, and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test. The microstructure was systematically investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved. Compared with T23 steel, there are fewer M23C6 particles at the grain boundaries in modified T23 steel, and the degree of intergranular weakening is smaller. At the same time, not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel, leading to the decline of intragranular strengthening. In addition, small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel. The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.

Prediction of Reheat Cracking Behavior in a Service Exposed 316H Steam Header

Reheat cracking in an ex-service Type 316H stainless steel steam header component has been investigated in this study. The examined steam header was in service for 87,790[Formula: see text]h and the cracks in this component were found in the vicinity of the weld toe. The root cause of this type of failure was due to the welding residual stresses. The welding-induced residual stresses had been present in the header at the early stage of the operation and were released during service. In this paper, a novel technique has been proposed to simulate the residual stress distribution normal to the crack direction by applying remote fixed displacement boundary conditions in an axisymmetric model. This approach can simulate the presence of residual stresses in actual components without the need to develop full weld simulation to quantify them. The predicted residual stress levels and distributions normal to the crack direction have been found in good agreement with the measured residual stresses available in the literature for a similar header. The creep crack growth (CCG) rates have been characterized using the fracture mechanics [Formula: see text] parameter and estimated using predictive models.

Welding Process Improvement of 347H Stainless Steel Boiler Tube for Mitigating Reheat Cracking

Many failures at the welded joint of 347H tubes have recently occurred during the short-term operation of the boiler tubes. To mitigate reheat cracking or strain induced precipitation hardening, the finite element analysis and experiment for optimization of welding procedure were performed. Preheating, high inter-pass temperature, low and high heat input conditions were compared with existing welding procedure. According to mechanical analysis, high inter-pass temperature and high heat input condition showed the lower plastic strain and tensile residual stress at the inner area of heat-affected zone. At high temperature tensile test, the high inter-pass temperature condition had lower yield strength and higher elongation, while the high heat input condition showed higher yield strength and smaller elongation. In the specimen of high heat input condition, the dissolution of most precipitates and coarse grain were observed in the heat-affected zone. Relatively small amount of dissolution and smaller grain got shown in high inter-pass temperature condition. Therefore, high inter-pass temperature condition can be an alternative. Through X-ray diffraction analysis, it was confirmed that nitrogen was added to Super304H and carbon was substituted for the existing crystal structure of Nb(C,N). This will mitigate reheat cracking by increasing the solvus temperature of Nb(C,N) and reduce the grain coarsening. Key words: Strain induced precipitation hardening, Carbide dissolution, Plastic strain, Heat input, Inter-pass temperature

Method and Criteria to Evaluate Reheat Cracking Susceptibility

Due to a lack of facile methods for evaluating the reheat cracking susceptibility of the coarse grain heat-affected zone (CGHAZ), a simple notched C-ring reheat cracking test method and evaluation criteria were proposed. Accordingly, a suitable C-ring sampling method and heat treatment procedure were established. The C-ring specimens were taken from thick plate joints welded under five different heat inputs and testing was carried out. Reheat cracks were observed and identified under different stress loadings. Subsequently, related with the evaluation criteria of the Gleeble® test, the evaluation criteria based on notched C-ring tests were developed. This method can reflect both the microstructure and restraint stress of actual welded joints, and the stress relaxation during heat treatment. Moreover, the experimental operation is simple and repeat-able. It is expected that an evaluation standard for reheat cracking sensitivity in CGHAZ will be established.

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Evaluation of Reheat Cracking Susceptibility in High Strength Austenitic Stainless Steels

Mechanism of reheat cracking in electron beam welded Ti2AlNb alloys

Abstract In order to clarify the characteristics and formation mechanism of the reheat cracking in Ti2AlNb weldments, a series of heat treatment conditions were performed to the circular joints welded by electron beam, and then the macrostructures and microstructures were investigated using optical microscopy, scanning electron microscopy, X-ray diffractometry, and transmission electron microscopy. The results show that the reheat cracking occurs primarily along the grain boundaries in the weld when the Ti2AlNb circular welded joints are heated up to about 700 °C. During the heat treatment, an almost complete transformation of B2→O happens while the temperature goes up through the O single-phase region. Then, O→B2+O phase transformation occurs primarily along the grain boundaries as the weld metal continues to heat up to the B2+O dual-phase region. Under the high tension stress consisting of welding residual stress and phase transformation stress, reheat cracking occurs at the interface between the B2+O dual-phase layer and the O-phase matrix.

Reheat Cracking Susceptibility in Coarse Grained Heat-affected Zone of SA508-4 Steel for Nuclear Pressure Vessel

Reheat Cracking Susceptibility in Coarse Grained Heat-affected Zone of SA508-4 Steel for Nuclear Pressure Vessel

From the Field to the Lab: Real Scale Assessment of Stresses in Welded Components

Abstract Residual stresses are crucial when assessing the performance of welded components. The present work deals with the possibilities of transferring the real-life boundary conditions of welding, which influence the residual stress, into the laboratory. The possibilities of a test system specifically developed for this purpose, with a maximum capacity of 2 MN, are shown, because the structural design, global process, geometry, and material-dependent stresses are induced, which can be simulated and quantified within the system. Additionally, X-ray diffraction can be applied to determine the resulting local stress distribution precisely with high spatial resolution. Two examples are presented to show how the conditions to be found during production are simulated in the laboratory. It is shown how welding stresses in high-strength steels are affected by the heat control. It was possible to clarify why elevated working temperatures significantly increase the bending stresses in the welded joint and therefore the tensile stresses in the heat-affected zone. The effect of heat treatment applied under stresses resulting from welding is demonstrated by the example of a creep-resistant steel. Reheat cracking is significantly increased in this case, as compared to small-scale laboratory-based tests.

Evaluation on Reheat Cracking of Multi-pass Weld Metal of YP 690 QT Steel

This study evaluated the relationship between reheat cracking that occurs in the reheated weld zone formed by subsequent multi-pass welding and the preheat/interpass temperature. The multi-pass weldment was YP 690 MPa high-strength steel, which is used in mobile offshore platform units. The welding process was flux-cored arc welding (FCAW), which coupons were constrained by welding to simulate an actual constraint condition. Three preheat/interpass temperature ranges were employed. The characteristics of the weldment subjected to preheat/interpass temperatures higher than 80°C satisfied the relevant specifications. In the bending testing, a weldment subjected to a preheat/interpass temperature range of 50-60 °C was fractured. The results demonstrated reheat cracking in the reheated weld zone in multi-pass weldments. Reheating cracking (RHC) was initiated at carbides precipitated along the grain boundaries, induced by multipass heating; some cracks were arrested in the reheated zone and the others propagated to the columnar weld metal. The hardness of the cracked specimens was higher than that of the specimen subjected to a higher preheat/interpass temperature, confirming that a susceptible microstructure had formed due to the faster cooling rates. In the analysis of weld residual stress, the specimen with the lower preheating/interpass temperature range showed higher tensile residual stress than that of high preheating/interpass temperature range. Therefore RHC in the reheated zone of the weldment was mainly affected by grain boundary carbides, a higher hardness, and residual stresses. Based on the above discussion, the mechanism of RHC and assessment of structural integrity is presented. Key words: flux cored arc welding, reheating cracking(RHC), preheat/interpass temperature, quenched-tempered(QT), residual stress, structural integrity

Effect of heat treatment on fatigue crack initiation of laser powder bed fusion stainless steel 316L

Laser powder bed fusion (L-PBF) is an emerging additive manufacturing technique for building structural components. L-PBF processing defects, such as lack of fusion pores, promote fatigue crack initiation and shorten the fatigue life. With well-controlled processing, critical pores can be avoided such that the microstructure-driven intergranular crack initiation mode becomes operative. In this work, the fatigue crack initiation behaviours of as-built and solution annealed L-PBF stainless steel 316L were studied. Crack initiation of the as-built samples is driven by de-bonding of the dendritic grain boundaries. High temperature annealing results in the formation of thermally-induced defects, possibly via the reheat cracking mechanism and the nucleation of pre-existing gas pores. As heat treating could have led to recrystallization and annihilation of the original grain boundary defects, the thermally-induced defects became the new sites for crack initiation. In addition, heat treatment incurred significant reduction in yield strength, such that the interaction of fatigue and ratcheting strain accumulation dominated the deformation behaviour of the material. The resulting fatigue strength in the finite life regime was reduced by about 13% but the fatigue endurance limit was not affected.

Prospects for using laser radiation to improve cracking resistance in welding cryogenic steels

It is shown that in welding large structures with stable austenitic welding materials made of nitrogen-containing cryogenic chromium–nickel–manganese austenitic steels, cracks can form in the welded joint both during crystallization and in the solid-state at subsolidus temperatures. In laser welding, the structure of the cast weld metal can be greatly refined in comparison with arc welding and this should increase the solidification cracking resistance. In addition, laser welding can be used to produce the welded joint in a single pass thus preventing reheat cracking.