Local Post Weld Heat Treatment Research Articles

Correlating pipe dimensions and success of local heat treatment: Developing nomograms to deduce heat treatment parameters

Local post-weld heat treatment (PWHT) is the only option for heat treating field welded joints. Quite often, the success of local PWHT in alleviating the residual stress and tempering the material within the soak band (SB) is dependent on the ability to achieve the required heat treatment temperature and maintain through-thickness temperature gradient (TTG) within the specified limits at the end of heating cycle, whence soaking begins. Field observations reveal the inadequacy of AWS D 10.10 specified parameters viz. rate of heating (ROH), heat band (HB) and insulation band width in not achieving the required TTG for certain pipe dimensions. Although prior works have attempted to address this issue by widening the HB, the capacity of the heating equipment often pose a limitation. In such cases, reducing ROH is a plausible alternative. With, no such prior studies seen in literature, an exhaustive finite-element analysis simulating the local PWHT on 81, SA106GrC pipe samples (diameter and thickness varied in 9 levels each) was performed, thrice. First as per AWS recommendations, second by halving the code deduced ROH and third by doubling the code deduced HB. The trend of important outcomes (TTG, power source rating and energy consumption) with great significance to the heat treatment industry were also compared and analysed. Two nomograms were developed to serve as a ready reckoner for field heat treater in not only assessing the adequacy of heat treatment parameters but also with possible alternatives in achieving the desired TTG using field-available power source.

A Uniformed Calculation Criterion on Heat Band Width of Local PWHT on Welded Joint with Dissimilar Thickness

Local post-weld heat treatment is used to reduce welding residual stresses. The existing standards have great differences in the selection of the width of the heated band, and the heating width, as an important control parameter of the local heat treatment, will directly affect the quality of the heat treatment. In this paper, the numerical simulation method is used to simulate the welding and heat treatment process of unequal-thickness joints. The stress and deformation of the joint with different thickness ratios under different heating widths are studied by finite element simulation, focusing on the influence of the width of the heated band on the residual stress relief of the joint. Based on these studies, the criteria for determining the optimal width of the heating zone are consistent. Finally, the formula HB=HB1+HB2=3RT+1+k2RT for calculating local heat treatment heating width based on the thickness of welded joint for SA738Gr.B steel is established. Among them, HB1 is the width of the main heating zone, HB2 is the width of the auxiliary heating zone, k is the thickness ratio of the thick plate to the thin plate, and t is the wall thickness of the thin plate.

Thermo-Mechanical Study of TIG Welding of Ti-6Al-4V for Residual Stresses Considering Solid State Phase Transformation

To overcome the detrimental effect of residual stress in welded joints, which affects the overall performance of the welded structure, this paper studies the magnitude and distribution of residual stress after welding and local post-weld heat treatment (PWHT). The coupled thermo-metallurgical-mechanical model for welding 6 mm thick Ti-6Al-4V (TC4) titanium alloy plates was established, the evolution of the SSPT and its effect on the residual stress were quantitatively analyzed, and a parametric analysis of local PWHT was performed. The results demonstrated that there was good agreement between the numerical results and the experimental data. Due to the cooling rate reaching 327 °C/s, the volume fraction of α、 in the fusion zone (FZ) reached 0.218 after welding and decreased by 90.83% after PWHT when the heating temperature was 700 °C. The peak value of the longitudinal residual stress can reach 686.4 MPa after welding with SSPT, which was 11.38% lower than that without SSPT, and it decreased by 65.6% after PWHT when the heating temperature was 900 °C. The research results demonstrate that SSPT has a significant effect on residual stress, and PWHT can obviously reduce the residual stress, which provides a certain reference for welding TC4 titanium alloy plates.

Optimization of Temperature Field by Differentiating Number of Circumferential Control Zones in Local Post Weld Heat Treatment on 9%Cr Heat-Resistant Steel Pipe

Abstract This paper reports the optimal use of control zones to achieve required uniformity of temperature distribution in local postweld heat treatment of welds in 9% Cr heat-resistant large pipe system. In this research, local post weld heat treatment tests on temperature distribution with different control zones were carried out on 9%Cr steel pipes (OD 710 mm × 35 mm and OD 575 mm × 35 mm), which was further used for the development of the thermal analysis of the postweld heat treatment model via abaqus. The research results revealed (1) the effect of number of control zone on the uniformity of temperature distribution; (2) the effect of size of pipe diameter on the circumferential temperature and the through-thickness temperature gradients. This article discusses the possible reasons for the temperature difference at various positions of the pipe caused by the air flow inside the pipe with different control zones. Based on the obtained results, a practical method was designed for the selection the number of circumferential control zones on 9%Cr heat-resistant steel pipeline according to the required degree of temperature distribution uniformity. This paper contributes to the specific knowledge and the generic methodology.

Reducing Full-Field Residual Stress of Girth Weld With Thick Wall by Combining Local PWHT and Water Jet Peening

Abstract In this paper, a new controlling method combined local postweld heat treatment (PWHT) and water jet peening (WJP) was proposed to reduce full-field residual stress of girth weld with thick wall. The finite element analysis with sequential coupling method and experiment measurement were performed to study the residual stresses distribution before and after the new controlling method. The results show that the hoop and axial stresses through the thickness are all decreased greatly by using the new controlling method. As long as the new controlling method is used with appropriate parameters, the residual stresses on inner surface can be reduced from tensile stresses to compressive stresses, which greatly improve the resistance ability to stress corrosion of inner surface.

Small Diameter Tube Post-Weld Heat Treatment Process with Brinell Hardness and Microstructure Inspection

The local post-weld heat treatment (PWHT) process test of 9Cr-3W-3Co small-diameter pipe welded joints was carried out using rope-type resistance heaters with different parameters, and the Brinell hardness test and microstructure observation were carried out on the welded joints after heat treatment. The results show that when the heating width was 200mm, the constant temperature was 790°C-800°C, and the constant temperature time was 2 hours, the Brinell hardness of the weld was in the range of 246HBW-265HBW, which had good performance. After tempering at 760°C-800°C, the welds all showed a clear tempered lath martensite.

An Economical and Mechanical Investigation on Local Post-Weld Heat Treatment for Stiffened Steel Plates in Bridge Structures

A heat treatment is effective for reducing the residual stress of the welded structures. A post-weld heat treatment (PWHT) requires a large heating apparatus (furnace). It requires a high energy, a long time, and a high cost. For examining the possibility of cost and energy saving in PWHT work, an economical and mechanical investigation of the local PWHT to stiffened plate members in steel bridges was conducted. The expense of apparatus for the furnace PWHT was 1.5 times higher than that of local PWHT by sheet-type ceramic heaters. When the number of heater units was reduced and were repeatedly used, the expense for the apparatus became lower. However, it took longer to complete the heat treatment than with the furnace PWHT or the local PWHT with full heater units. The thermal elastic-plastic finite element (FE) analysis examined the effect of local PWHT. The tendency of the stress distribution after the local PWHT differed from the welding residual stress or the stress after the furnace PWHT because of the temperature difference between the heated and the non-heated parts of the local PWHT. However, the effect of residual stress relief by the local PWHT could be almost the same as that of the furnace PWHT.

Numerical and experimental investigations on the temperature field in local post weld heat treatment of 9 % Cr heat resistant steel welded pipes

Local post weld heat treatment (PWHT) is usually employed in the field fabrication of large-sized 9% Cr heat resistant steel welded components, such as power plant boilers. Temperature field in local PWHT plays critical roles in both achieving PWHT purposes and avoiding detrimental effects of local heating. The objective of this research is to quantify the effects of local PWHT parameters and pipe dimensions on the temperature field in local PWHT of 9% Cr heat resistant steel pipes and finding the optimal process window to engineer desired weld properties. Finite element (FE) models were developed to simulate the temperature field of local PWHT and carefully verified by eight experiments using four 9% Cr heat resistant steel pipes with typical dimensions employed in ultra-supercritical (USC) boilers. The results show that the through-thickness temperature gradient is severely aggravated with the increase of pipe diameter and wall thickness, which will lead to inadequate tempering in the weld metal close to the inside surface of the pipe. To enlarge heated band (HB) width is an efficient way to mitigate the through-thickness temperature gradient, and the axial temperature gradient can be effectively relieved by augmenting gradient control band (GCB) width. The through-thickness temperature gradient is the major factor in implementing successful local PWHT for 9% Cr heat resistant steel pipes. A much smaller process window is found in 9% Cr heat resistant steel pipes due to their higher PWHT temperatures and narrower range of permissible PWHT temperatures.

A primary plus secondary local PWHT method for mitigating weld residual stresses in pressure vessels

Local post-weld heat treatment (PWHT) is often used for mitigating welding-induced residual stresses in pressure vessels and piping systems. As large and ultra-large pressure vessels are increasingly used in petrochemical and power generation industries, traditional local PWHT procedures stipulated in Codes and Standards become difficult to implement for some of the ultra-large vessels due to their very large diameter and overall length. In this paper, a new method referred to as primary-secondary heating based local PWHT technique (PS-PWHT) is proposed. Both finite element residual stress modeling and experimental residual stress measurements were performed for both validating the effectiveness of the proposed technique and elucidating the underling mechanisms. The results show that the PS-PWHT method can reduce welding-induced residual stresses in a significant manner, even leading to some level of compressive residual stresses at the vessel weld region.

Using reinforce plate to control the residual stresses and deformation during local post-welding heat treatment for ultra-large pressure vessels

Larger residual stresses and deformations are inevitably generated during local post-weld heat treatment (PWHT) for ultra-large pressure vessels, which may lead to cracking in weld joint. In this paper, a new control method with reinforced plate was proposed to prevent cracking during local PWHT for ultra-large pressure vessels. And the evolution of stresses and deformation during local PWHT and the reason of cracking were discussed. The results show that bigger inconsistent deformation along axial direction and larger axial stresses in weld toe are the main reason to cause cracking. The hoop, axial and maximum principal stresses in welded joint can be reduced by reinforced plate. And the maximum radial deformation during holding stage of PWHT was decreased by reinforced plate because of the decreased elastic and plastic strain. The maximum radial deformation was decreased by more than 50%. The inconsistent deformation at heating stage can be improved, so the stress concentration in weld toe resulting in cracking could be also improved. This work will provide a good guidance for the manufacture of ultra-large pressure vessels.

Influence of local post-weld heat treatment and its thermal analysis on thick wall carbon steel pipe

Local post-heating was a common process during the field fabrication and repair of engineering components. Post Weld Heat Treatment (PWHT) attempts to relax residual stress when service requirement dictates the need for PWHT. The additional objectives such as hardness reduction stress relaxation aimed to be below a specific threshold level. Many factors have influenced on PWHT procedures, such as the size of the pipe, heating width, insulation conditions, heating rates, soaking temperature and holding time, material composition, etc. However, up to now, the influences of these factors on PWHT were not clearly understood and different criteria for sizing the parameters. It can be found in different codes. PWHT was to attempt and duplicate the outcome of furnace heating (i.e heating the whole weldment) within a localized region, referred to the soak band surrounding the weld. The width of heating coil from 500 to 600 mm significantly reduced the temperature gradient in between OD and ID of the carbon steel pipe. The increase in thermal energy moderately enhanced the temperature gradient on the carbon steel pipe.

Effect of Internal Air Flow on Local Postweld Heat Treatment for Large Diameter ASME P92 Steel-Welded Pipes

Abstract Local postweld heat treatment (PWHT) is usually employed in field fabrication of large-sized ASME SA-335 Grade P92 steel pipes. Internal air flow in pipes that arise from field fabrication can result in considerable convection losses on the inside surface of the pipe when the pipe is not strictly sealed off. Welding and local PWHT experiments of a large diameter P92 steel pipe were conducted both with and without internal air flow, and temperature field of both sides of the pipe was measured. The conjugate heat transfer between the pipe and the internal air is simulated using computational fluid dynamics (CFD) method. The effect of internal air flow on temperature field was further investigated. Results indicate that temperature gradient along through-thickness direction and axial direction during local PWHT is significantly increased due to internal air flow. The increasing rate of temperature difference between inner and outer surface at weld centerline to internal air velocity is about 14.5 °C/(m s−1). The maximum temperature is no longer located at the weld centerline, which will lead to a risk of overheating. The temperature drop is severer in the air inlet side than air outlet side at same distance from weld centerline. For local PWHT to be successful, the internal air flow should be strictly limited during local PWHT; otherwise, the width of heated band (HB) should be extended.

Finite element analysis of residual stress in 2.25Cr-1Mo steel pipe during welding and heat treatment process

The circumferential butt-welded pipe was fabricated using 2.25Cr-1Mo steel and then subjected to local post-weld heat treatment (LPWHT) for releasing residual stress after multi-pass welding. The sectioning method was employed to measure residual stress. Based on SYSWELD software, a thermo-metallurgical-mechanical coupled computational approach was developed to analyses the residual stress distribution on 2.25Cr-1Mo welded pipe. A three-dimensional finite element model and moving heat source model were employed in simulation. The effects of solid-state phase transformation (SSPT), tempering as well as creep were considered simultaneously. The effectivity of computational approach was verified by experimental measurement. The simulation results showed that SSPT could induce an undulating stress profile and large stress gradient in welded zone. The stress gradient could be reduced during LPWHT while there were stress peaks at the edges of heated region. Tempering effect could decrease the residual stress at high-stress region by material softening and yielding. Creep caused a global transformation from elastic strain to plastic strain, which led to a macro-scale stress relaxation. It is necessary to consider SSTP, tempering as well as creep in material model for more accurate simulation results.

Effect of Pipe Length on the Temperature Field for 9% Cr Heat-Resistant Steel Pipes in Local Post Weld Heat Treatment

ANSYS was used to establish the calculation model of the temperature field for 9% Cr heat-resistant steel pipes in local post-weld heat treatment (PWHT). And the computation model has been validated experimentally. Based on the numerical model, the temperature field distribution was simulated for different pipe lengths in local PWHT, and effects of pipe length on the temperature distribution of heat treatment was further studied. Results show that increase of pipe length has little influence on equivalent temperature measurement point, soak band (SB) width of outer wall and axial temperature gradient, while it causes temperature difference between outer and inner wall to increase and soak band width to reduce significantly in inner wall. And when pipe length increases to a certain extent, the temperature field tends to be stable gradually.

Improving the HAZ toughness of Q+T high strength steels by post weld heat treatment

Steel producers are continuously developing the mechanical properties and improving the weldability of high strength steels. Quenched and tempered steels belong to the one of the highest strength grades of structural steels with outstanding toughness characteristics due to the high temperature tempering. During fusion welding the thermal cycle irreversibly changes the microstructure and the mechanical properties of the base material, therefore an inhomogeneous heat-affected zone (HAZ) forms. In the HAZ hardened and softened zones occur. Due to the thermal cycles experiences during welding, these HAZs can exhibit significant losses in toughness when compared to the base metal. In real welded joints the HAZ properties can be analysed by conventional material tests to a limited degree; therefore physical simulators have been developed for the detailed examination of different HAZ areas. In the present research work the HAZ properties of a 960 MPa yield strength quenched and tempered steel (S960QL according to EN 10025-6) are investigated. Since the toughness reduction can be only partially handled by the adjustment of welding parameters, the possibility of local post-weld heat treatment was examined. Based on preliminary physical simulations and welding experiments a medium heat input gas metal arc welding technology (t8/5 = 15 s) was selected for the HAZ simulations. The welding and the post-weld thermal cycles were determined according to the Rykalin 3D model. The effect of post weld heat treatment on the properties of the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones were investigated by electron microscopic, hardness tests and instrumented Charpy V-notch impact tests. The materials tests showed significant improvement of the toughness properties especially in ICHAZ due to the post-weld tempering, whilst the softening was acceptable.

Effect of Local Post Weld Heat Treatment on Tensile Properties in Friction Stir Welded 2219-O Al Alloy

To improve the formability of the aluminum alloy welds and overcome the size limitation of the bulk post weld heat treatment (BPWHT) on large size friction stir welded joints, a local post weld heat treatment method (LPWHT) was proposed. In this method, the resistance heating as the moving heat source is adopted to only heat the weld seam. The temperature field of LPWHT and its influence on the mechanical properties and formability of FSW 2219-O Al alloy joints was investigated. The evaluation of the tensile properties of FSW samples was also examined by mapping the global and local strain distribution using the digital image correlation methodology. The results indicated that the formability was improved greatly after LPWHT, while the hardness distribution of the FSW joint was homogenized. The maximum elongation can reach 1.4 times that of as-welded joints with increase the strength and the strain of the nugget zone increased from 3 to 8% when annealing at 300 °C. The heterogeneity on the tensile deformation of the as-welded joints was improved by the nugget zone showing large local strain value and the reason was given according to the dimple fracture characteristics at different annealing temperatures. The tensile strength and elongation of LPWHT can reach 93.3 and 96.1% of the BPWHT, respectively. Thus, the LPWHT can be advantageous compared to the BPWHT for large size welds.

Influences of local PWHT from different criteria at home and abroad on the residual stress of the under-matching welded joint

The under-matching welded joint is more commonly used in engineering practice now and the post-weld heat treatment (PWHT) is the common method to reduce the residual stress of the welded joint, while the research on the under-matching welded joint after PWHT is few. And the different influences of the local PWHT from different criteria haven't been specifically analyzed by the researcher. The finite element software-ABAQUS is used to simulate the welding process and PWHT process of the under-matching welded joint which is the combination of CF62(base metal) and 316L(weld) in this paper. The discussion of the influences of local PWHT from different criteria at home and abroad on the residual stress of the under-matching welded joints are based on the experimental verification and the proposal criterion has been given in this paper. The numerical simulation by finite element has been proved to be useful to predict the residual stress of the under-matching welded joint after local PWHT from different criteria by the comparison between residual stress results of experiment test and numerical simulation in this paper. The local PWHT method of ASME-Ⅷ is the best and it is recommended in this paper. The criteria PD5500 and EN13445 are at the second place while the old criterion GB150-1998 is conservative. The new criterion GB150-2011 have been promoted very much than the old criterion GB150-1998 of the local PWHT method.

Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. All pipe spools were fabricated using identical pipeline production procedures for manufacturing multipass narrow gap welds. Nondestructive neutron diffraction (ND) strain scanning was carried out on girth welded pipe spools and strain-free comb samples for the determination of the lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses were measured on two pipe spools in as-welded condition and two pipe spools after local and furnace PWHT. Measurements were conducted through the thickness in the weld material and adjacent parent metal starting from the weld toes. Besides, three line-scans along pipe length were made 3 mm below outer surface, at pipe wall midthickness, and 3 mm above the inner surface. PWHT was carried out for stress relief; one pipe was conventionally heat treated entirely in an enclosed furnace, and the other was locally heated by a flexible ceramic heating pad. Residual stresses measured after PWHT were at exactly the same locations as those in as-welded condition. Residual stress states of the pipe spools in as-welded condition and after PWHT were compared, and the results were presented in full stress maps. Additionally, through-thickness residual stress profiles and the results of one line scan (3 mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry's R6 procedure. The residual stress profiles in as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

Local Postweld Heat Treatment of Heterogeneous Welded Joint

A concept of heterogeneous welded joint is proposed for researching and describing the characteristics of local postweld heat treatment (PWHT) temperature field for asymmetric thermal conductivity welded joint. Its three types have been classified according to thermal conductivity direction around the weld, separately, welded joint with transverse unidirectional thermal conductivity, transverse bidirectional thermal conductivity, transverse and longitudinal thermal conductivity. Compared with the temperature field of symmetric thermal conductivity welded joint, the highest temperature point of heterogeneous welded joint deviates from the heat device center, and uniform temperature area shrinks. In addition, longitudinal temperature difference and dramatic temperature change zone have arisen for the third type heterogeneous welded joint. In order to improve the temperature distribution, two PWHT methods called temperature compensation method and power compensation method have been put forward and developed. Several engineering applications of two methods are illustrated as examples.

A combined computational and experimental study on vibration stress relief for large welded DH36 steel tube

Vibration stress relief (VSR) is an effective and economic method for reducing residual stress in various welding components. Compared with other methods, it costs much less time and energy. In this study, finite element method (FEM) was used to assist VSR treatment of large DH36 steel welded tube by determining the 1st order vibration mode of the tube and the natural frequency of the tube in a hypothetic zero-stress state. According to the computational results, proper vibration exciting assembling and excitation strategy was selected. An effectiveness index, η, for fast and quantitative estimation of the residual stress decrease rate was proposed. η is determined by 1st order natural frequencies of the tube in three states, i.e. as-welded, VSR treated and a hypothetic zero-stress state. η in this study was 49.8 %, meeting well with the experimentally measured residual stress decrease rate, ~50 %. Thus, the validity of the effectiveness index was verified. This study provides a novel method for analysis of VSR effectiveness. In comparison, maximum residual stress was reduced by 20-57 % when traditional local post weld heat treatment (PWHT) was used. This indicates that VSR is a good stress relief method for DH36 welded structures.