Heat Affected Zone Regions Research Articles

Creep Rate, Friction, and Wear of Two Heat-Affected Zone Regions of 9–12 wt.% Cr Steels

Coarsening of precipitates can have a profound effect on the mechanical properties of martensitic 9–12 wt.% Cr steels, which are typically used in critical parts of fossil-fuel power plants such as turbines, headers, and main steam pipes. In the present study, changes in precipitates’ size and distribution in the simulated heat-affected zone of two different 9–12 wt.% Cr steels (X20 and P91) after different aging conditions were analyzed and correlated with their creep, friction, and wear behaviors. It was shown that prior to aging, the morphology of the steel matrix (prior austenite grain size and microstructure homogeneity) governed the creep rate and the tribological performance of both steels, while after aging their response was additionally determined by the combination of the number and the size of precipitates. For the selected samples (prepared under identical conditions), number of precipitates was found to be within a narrower range for the X20 steel as compared to the P91 steel. For both steels, aging for a shorter time at the higher temperature yielded significantly higher stationary creep rate values as compared to aging for longer time at the lower temperature. The increase was more pronounced in the P91 than in the X20 steel. Both prior to and after aging, the P91 steel typically provided slightly higher creep resistance than the X20 steel, while the latter provided slightly better tribological performance. Furthermore, as a function of the increasing number of precipitates, static coefficient of friction in air atmosphere was approximately linearly decreasing, while the wear rate initially decreased.

A study on HAZ behaviour in 800 MPa cold rolled and hot rolled steel weld

Abstract In the automotive sector, the demand for advanced high strength steels (AHSS) is increasing day by day. Based on the application, cold rolled and hot rolled steels are used for various components in a vehicle body. Typically, cold-rolled grades with dual-phase, DP780 steel is used in the form of welded blanks. Hot rolled grades with Ti-Nb microalloy content, like HS800 steel are used in as long members. Welding is an important step to be considered in the design of materials for mass production as required in the automobile sector. In this investigation, Pulsed Gas Metal Arc Welding (P-GMAW) is carried out on both steels with a solid filler wire of strength 800 MPa. Static tensile tests indicated that failure in both steels welds occurred in the heat-affected zone region. The crack initiation and propagation behaviour were compared in both steels. In DP780 steels, the presence of the acicular ferrite and acicular martensite resisted the crack initiation and propagation in the weld region whereas, the chaotic nature of the acicular ferrite in HS800 steel. Microstructural studies revealed that the reason for heat affected zone (HAZ) failure in HS800 steel is due to the presence of hard TiN particles with a size of more than 1 μm which causes decohesion in the matrix. In DP780 steel failure is due to the presence of tempered martensite in the subcritical heat affected zone (SCHAZ). This study divulges the influence of filler wire chemistry, dilution, and welding parameters on cold rolled and hot rolled AHSS steels used in the automotive industry.

Destructive testing and hardness measurement of resistance stud welded joints of ASTM A36 steel

Resistance stud welding process is mostly used in several industries for manufacturing and joining of structural parts and components. During this process, the stud is stably welded on a base plate or component before bringing the other part or component to join them together by means of mechanical fastening. Resistance stud welding is a fast process with high energy density and low heat input that minimize the amount of residual stress and is suitably used for welding and joining of sheet metals that have a risk of distortion which can cause problems during assembly steps. In this study, we investigated the optimum resistance stud welding process parameters for stud welding on 3 mm thick ASTM A36 steel. The variable process parameters include main current time (MC time) from 5 to 50 ms, and pre-current time (PC time) of 20, 40, and 60 ms. After stud welding is complete, destructive testing was performed on stud welds by visual inspection and bend test to primarily check which parameter combinations are suitable for stud welding of ASTM A36 steel. Then, the specimens were macro-tested and hardness test to check the soundness and changes in mechanical properties in the stud heat affected zone regions. It was found that for stud welding of ASTM A36 steel, the suitable parameter is MC time between 25 and 35 ms and PC time between 20 and 60 ms.

Proučavanje faznih transformacija u zoni uticaja toplote aluminotermijski zavarenih čelika za šine eksperimentalnim metodama i simulacijom

Rail transportation has been an emerging and promising transportation method for economic development in Thailand and several countries. Thermite welding is one of the major welding and joining processes used to weld rail steel both during construction and maintenance. Unlike most welding processes in which heat is generated by electrical energy, thermite welding use heat generated during the chemical reactions between iron oxide and aluminum or other metallic compounds to create the weld. The amount of heat generated is thus depended on the composition and ratios of iron oxide, aluminum, as well as other metallic compounds mixed in the thermite powder. In addition, the size, shape, and material used for thermite mold could also play an important role in the heat transfer process during thermite welding of rail steel. In this research, SYSWELD software developed by ESI Group is used to perform thermal-mechanical-metallurgical welding simulation during thermite welding of rail steel. The current article presents that research methodology used to formulate the prediction of microstructure developed in the heat-affected zone regions of thermite welding of railway steel. It is noted that this work attempts to evaluate how preheat, heat generation during chemical reaction, and possible post-weld heat treatment could be performed to controlled the microstructure of pearlitic rail steel using SYSWELD software. The results of this on-going research will be used as the baseline for future development of structural integrity program for improving joining of rail steel such as the design and selection of welding processes and materials involved for rail construction, especially when appropriate grades and welding procedures of rail steels must be chosen and developed to withstand the actual loading conditions.

Microstructure and mechanical behavior of P91 steel dissimilar welded joints made with IN718 filler

The paper presents the detailed microstructure study and mechanical behavior of P91 welded joint produced by gas tungsten arc welding process (GTAW) using the dissimilar nickel grade superalloy Inconel 718 filler (ERNiFeCr2). The microstructural changes in the weld fusion zone (WFZ) and heat-affected zone (HAZ) were also studied for varying heat treatment conditions. The effect of the Inconel 718 filler on the tensile properties, hardness and Charpy impact toughness (CIT) of the welded joint was also studied for as-welded and heat-treated conditions. The post-weld heat treatment (PWHT) was performed in a temperature range of 650–1060 °C. The microstructure study showed the formation of an unmixed zone at the interface of the HAZ and weld metal and remain unaffected during the PWHT. The region of weld metal near the boat fusion line showed the cellular and columnar dendrites. The center region of the bulk weld metal was solidified as the columnar dendrites and cellular dendrites along with segregation of the elements at inter-dendritic boundaries. The dendrite core showed the segregation of the Ni, and Cr; however, at dendrite boundaries, major segregation was observed for the elements Nb, Mo and Ti. The heat treatment up to a temperature of 810 °C has observed a negligible effect on the WFZ microstructure; however, a significant change in microstructure was observed for the P91 HAZ region. A significant change in the microstructure of the WFZ was measured after the normalizing at 1060 °C, followed by tempering at 760 °C for 2 h (complex heat treatment). The tensile strength of the welded joint was measured 817 MPa with a joint efficiency of 146% for as-welded condition and a maximum of 901 MPa with a joint efficiency of 161% for complex heat treatment. The welded joint CIT was measured a maximum of 48 J in as-welded condition and get reduced after the PWHT and measured a minimum of 35 J after complex heat treatment. A hardness variation was observed along the welded joint for as-welded and PWHT condition. In the as-welded condition, the hardness of the WFZ was measured about 417 ± 21 HV and showed a minute change with an increase in tempering temperature. The minimum hardness was measured in the inter-critical HAZ region for the as-welded joint and produce a significant level of heterogeneity in hardness variation along the P91 HAZ. PWHT has observed a considerable reduction in the hardness value of the P91 HAZs except for ICHAZ. A negligible level of hardness variation along P91 HAZ was measured after PW 4.

Evaluation of Stationary Creep Rate in Heat-Affected Zone of Martensitic 9–12% Cr Steels

The purpose of the present study was to evaluate the contribution of distinct regions of the simulated heat-affected zone (HAZ) to the overall creep behavior of welded joints in the X20 and P91 steels. The HAZ was simulated by means of dilatometry at four peak temperatures (900, 1000, 1200, and 1350 °C) with a consequent tempering at 650 °C. Microstructure features of the four simulated HAZ regions including precipitates, prior austenite grains, and subgrains were quantified by means of electron microscopy. The quantified parameters and the measured hardness were used in three physical models for evaluation of the stationary creep rate (ε˙ at 170 MPa and 580 °C. The resulting ε˙ values fall within the range 10−8–10−7 s−1, being in good agreement with the experimental data with a similar thermal history, but an order of magnitude lower than the measured values for the parent metal of the studied steels (10−7–10−6 s−1). Depending on the model utilized, their output can be linearly related to hardness, subgrain size, or interparticle spacing. The model relating ε˙ to hardness was the most consistent one in prediction, being always lower for higher peak temperatures.

Study of microstructural transition in dissimilar weld joint from as-welded to PWHT condition using electron backscattered imaging

Transition occurring in microstructures of heat affected zone (HAZ) of a P91-316LN SS dissimilar weld joint was studied using electron backscattered diffraction (EBSD). Kernel misorientation angle and grain boundary character distribution maps were used to study local strain accumulation and evolution of low angle grain boundaries in the HAZ regions. Two microstructural conditions i.e. as-welded and post weld heat treated (PWHT) were investigated. It could be seen that PWHT reduced the strain gradient in the P91 steel’s HAZ which was formed during welding and that localised softening occurred within this region after PWHT.

Welding thermal stress diagrams as a means of assessing material proneness to residual stresses

In this work, the proposal and appraisal of a method to describe in a quantitative manner the phenomenon of thermal stresses formation in welding at different heat-affected zone (HAZ) regions and under different cooling rates, by means of physical simulation, are explained. Under the denomination of welding thermal stress diagrams (WTSD), initially the concept and experimental arrangements needed to use the idea, based on a Gleeble simulator, are revealed. An approach to determine more realistic thermal cycles (peak temperature and heating/cooling rates) is introduced and applied. The method assessment was carried out by using specimens of a HSLA quenchable steel subjected to different cooling rates (covering a wide range of typical welding heat inputs) and peak temperatures (representing regions progressively farther away from the fusion line). The different thermal stress (TS) curves proved the concept based on the justification of the results. In addition, it was physically demonstrated that TS curves are governed mainly by two complex concurrent phenomena, namely contraction under restriction of heated areas and the expansibility of phase transformation. It was concluded that due to this balance, the highest residual stress (RS) does not occur either at slowest cooling rate or at fastest cooling rate. Nevertheless, the highest RS may not occur at the coarse grain zone either. TS progressively drops along the HAZ regions away from critical regions, and even at sub-critical regions there is tensile RS. Complementarily, it was also concluded that WTSD by physical simulation allows one to determine the deformation behaviour of a material as a function of temperature. This information can be used as input or calibration in modelling for thermal stress generation in steels.

Study on the influence of heat input on mechanical property and microstructure of weld in GMAW of AISI 201LN stainless steel

ABSTRACT Nitrogen alloyed austenitic stainless steels are popular among manufacturing industries due to its excellent mechanical properties and resistance to corrosion, used for the production of various components in food and petrochemical industries. Due to its improved strength and corrosion resistance, AISI 201LN replaces the commonly used SS 304 grade. In gas metal arc welding (GMAW) of AISI 201LN stainless steel the evaluation of heat greatly affects mechanical strength of the weld. In this work, the microstructure and mechanical strength of the weld were investigated in three different heat inputs viz., low, medium and high. The quality assessment of weld joint is examined by radiographic examination. The welded joint at low heat input was found to exhibit higher ultimate tensile strength and hardness compared to the welded joint at medium and higher heat input. The variation of hardness value for all three welded joint was found to increase from weld centre towards the base metal. The micro-structural examination shows finer morphology in heat-affected zone region in welded joint at lower heat input. The fine structure observed is due to higher cooling rate that causes restriction of grain growth.

EVOLUTION OF STRUCTURE, PHASE AND MECHANICAL PROPERTIES DUPLEX STAINLESS STEEL (DSS) 31803 WELDING USING GMAW WITH THE ADDITION OF CO2 TO Ar - CO2 SHIELDING GAS

The purpose of this study was to analyze the effect of CO2 on the composition of the Ar- CO 2 shielding gas in DSS 31803 duplex welding using GMAW on carbon deposition, ferrite fraction, microstructure, weld and HAZ mechanical properties. This study uses A (100% Ar), M1 (95% Ar + 5% CO 2 ), M2 (90% Ar + 10% CO 2 ), M3 (85% Ar + 15% CO 2 ), M4 (80% Ar + 20 % CO 2 ), M5 (75% Ar + 25% CO 2 ) and C (100% CO 2 ). From the results of the study, the microstructure of the weld metal in each specimen contained grain boundary austenite (GBA), Widmanstätten austenite (WA), intragranular austenite (IGA), and partial transformed austenite (PTA). Scanning electron microscopy (SEM) identifies the occurrence of chrome carbide precipitation (Cr23C6) HAZ region in specimens M5 and specimen C. The equilibrium phase of ferrite and austenite in weld metal areas is fulfilled in specimens A (34.8), M1 (38.9), M2 (36.4), M3 (38.4) and M5 (45.9). The highest average hardness value of vikers in weld metal area is found in M5 specimen which is 255 HV. While the lowest value in specimen C is 238 HV.The highest Vickers average hardness value in the heat affected zone (HAZ) is in specimen A which is 271 HV and the lowest hardness is in specimen C which is 245 HV. High tensile strength values are found in specimen M5 with value 800 MPA. While the value of low tensile strength in M4 specimens which is 652 MPA.

Characterization of structure-property relationship of martensitic P91 and high alloy ferritic austenitic F69 steel

The present work aims to investigate the microstructure and mechanical behavior of the dissimilar welded joint (DWJ) of martensitic A 335-grade P91 steel and high alloy ferritic austenitic A 182 F69 steel produced by the gas tungsten arc welding (GTAW) process. The microstructural studies showed the variety of the microstructure across the DWJ, which formed mainly due to the solidification and solid-state phase transformation as a result of the weld thermal cycle. The microstructural studies were performed in as-welded as well as post-weld heat treatment (PWHT) conditions. PWHT has been conducted to improve the mechanical properties of the P91 heat-affected zone (HAZ) at 760 °C (PW 1) and 810 °C (PW 2) for 2 h. For the mechanical properties of the DWJ, tensile tests, and microhardness tests were conducted. The fractured tensile specimen has been characterized using the scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The centre region of the weld metal showed the equiaxed dendrites while the interface near the boat fusion line, on both the P91 and F69 sides, showed the cellular and columnar dendrites. PWHT has observed a negligible effect on the microstructure and hardness of WFZ and F69 HAZ, while a significant change was observed for P91 HAZ. The tensile strength of the DWJ was measured maximum (735 MPa) for as-welded condition and reduced after the PWHT and for PW 2, it was measured lower than that of P91 and F69 base metal. The failure of the tensile specimen was also noticed in the soft inter-critical HAZ (ICHAZ) region for PW 2 condition.

Failure Analysis of a Flare Tip Used in Offshore Production Platform in Qatar.

An immature failure of a gas flare tip used in Qatar oil and gas offshore industry was investigated throughout this study. The design lifetime of the flare was fifteen years; however, it manifested immature failure resulting in a reduction of its lifetime to ten years. The flare is composed of different parts where the upper flare body and wind deflector showed failure while other components were still healthy. The material used for the aforementioned failed parts was Incoloy 800H, which is a highly corrosion and high-temperature resistant steel alloy. The material was rolled up and welded together with different welding joints. The root cause of failure was identified by using chemical analysis and microstructural and mechanical characterizations. For the mechanical characterization, an optical microscope (OM) and scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) analyses were used for the specimen extracted from the failed part in order to ensure that the material mentioned by the manufacturer demonstrated the same metallurgical properties. For the mechanical characterization, two sets of specimens were used, one close to the failure region and the other far from the failure area. The chemical analysis revealed that the material was truthfully Incoloy 800H. The mechanical examination results showed a significant reduction of mechanical properties, i.e., the ultimate tensile strength (UTS) and microhardness dropped by 44% and 41% for samples close and far from the failure regions, respectively. Careful examination of the failed parts indicated that failure mostly took place in the vicinity of the welds, in particular near the joints. Improper joint designs, as well as a number of joints being designed in tiny areas, worsened the harmful effect of the heat-affected zone (HAZ), resulting in crack nucleation in the HAZ regions. The effect of welding in a combination of harsh service conditions of flare caused further crack extension where they merged, resulting in final immature failure.

Tracking local brittle zone in the heat affected zone of girth-welded API 5L x46 pipe

In this study, microhardness variation as well as macro and micro structural examination of the heat affected zone (HAZ) of a girth welded API 5L X46 pipeline material were conducted as a means of tracking local brittle zone (LBZ) in the HAZ region. The weldment analysed were built from heat input range of 695 J/mm – 2567 J/mm. Analysis of the results revealed that the HAZ profile changes with variation in the heat input and becomes shallow but wider as the heat input increases. Defects free welds were achieved under the heat input range of 1650 J/mm – 2017 J/mm welding condition. Localized high hardness values were obtained at certain locations within the HAZ of intermediate heat input welds produced at 1467 J/mm due to thermal stresses induced strains at this heat input in the resolidified weld. Other than this, non-equilibrium rapid heating/cooling that is common during welding as well as the magnitude of mechanical strain generated on cooling vary with heat input and was attributed to the development of high hardness value at localized region within the HAZ of the welds in low heat input welding condition. The macrographic profile at these locations, contrasted against that of a failed pipeline material of similar specification obtained from typical oil and gas infrastructure, established that crack initiation and propagation followed the trend of microhardness variations in the girth welded pipe. The crack initiates at specific location in the HAZ with very high hardness in the range 186-216 Hv within a radius of about 3-5 mm from the edge of the fusion zone.
 Keywords: API 5L X46; Girth welding; Heat input; Heat affected zone; Local brittle zones

Oxidation Behavior of Microstructurally Different Regions of TIG Weldment in ASTM SA 210 GrA1 Boiler Steel

Abstract ASTM SA 210 GrA1 steel is used for many different applications of steam-generating systems in thermal power plants because of its resistance to corrosion at elevated temperatures and low cost of production. This study examines the oxidation behavior of tungsten inert gas (TIG) weldment in GrA1 boiler steel. Different regions of the weldment were identified by optical microscopy. The oxidation studies were then performed about heat-affected zone (HAZ) and weld metal specimens of TIG weldment after exposure to air at 900°C under cyclic conditions. The thermogravimetric method was applied to observe the kinetics of oxidation. X-ray diffraction and scanning electron microscopy with energy-dispersive X-rays techniques were used to analyze the oxidation product. The HAZ region confirmed the presence of moderately more oxidation (in terms of weight gain of 145.045 mg/cm2) than that of weld metal (139.797 mg/cm2) because the formation of the thick porous oxide scale had internal voids to a high extent.

Mechanical and Microstructural Characteristics of Conventional and Robotic Gas Metal Arc Welded Low Carbon Steel Joints: A Comparative Study

In the recent automobile era, SAE 1022, low carbon manganese steel is widely used in structural members of automobiles. These steels possess good weldability and need proper heating treatment conditions when high thickness sections are being used in the structural application. Mostly, gas metal arc welding (GMAW) is preferred in automobile industries to have which produces better penetration with some minor defects like improper fusion of sidewall, etc. The increasing demand in the field of the automobile led to the attention over automated GMAW is being preferred to reach better productivity with zero defects. The samples were welded by the conventional GMAW method and automated through a robot. From the experimental test results, welding current of 450A, welding speed of 350 mm/min and a land height of 4 mm yielded a maximum tensile strength of 647 MPa and hardness of 275 HV in the coarse-grained heat-affected zone region with a robotic mode of GMAW. The comparative study proved that automated process produced better mechanical properties and improved efficiency.

Investigation of the Correlation between Hydrogen Cathodic Charging Conditions and Toughness Properties of Longitudinal Submerged Arc Welded X65 Pipeline Steels

This research work focuses on the investigation of the correlation between hydrogen cathodic charging conditions and toughness properties of a welded X65 pipeline steels. The experimental technique which was applied to determine the characteristic parameters associated with toughness was the crack tip open displacement after hydrogen cathodic charging process. The hydrogen cathodic charging process was carried out in an electrolytic cell with applied current densities of 10, 20, 30 mA/cm2 and hydrogen cathodic charging duration 48 h. Through this study, interesting results were provided concerning the diffusion depth and the diffusion coefficient of atomic hydrogen as far as toughness properties. The maximum diffusion depth value is assigned for each current density field in the region of heat-affected zone and the minimum in the area of fusion zone. In addition, the highest diffusion coefficients are attributed to heat-affected zone and the lowest to fusion zone, for all applied current densities. Finally, the fusion zone is considered to be more prone to the occurrence of embrittlement phenomena compared to base metal. The rate of toughness parameters drop is higher for current densities between 0-10 and 20-30 mA/cm2, where the cathodic current promotes the electrochemical reduction in hydrogen cations to atomic hydrogen.

Nanoindentation investigation on the creep behavior of P92 steel weld joint after creep-fatigue loading

Using nanoindentation technology, the creep behavior of P92 steel welded joint that had experienced creep-fatigue (CF) loading was studied at the nanoscale. In both as-welded and CF samples, the fine-grain heat affected zone region exhibited relatively lower hardness and lower creep resistance than other regions of the welded joint. Compared to the as-welded sample, enhanced creep deformation and reduced hardness were observed in the CF sample. The strain rate sensitivity was in the range from 0.010 to 0.035 for as-welded sample and 0.052 to 0.082 for the CF sample, as the measurement region changed from base metal to weld metal regions.

Fracture toughness evaluation for thin-shell stainless steel weldment

Integrity of spent nuclear fuel (SNF) interim storage canisters are very important to the safety of the back-end nuclear fuel cycle. Stress corrosion cracking (SCC) potential of interim storage canister has been considered as a high priority. Because no post-weld heat treatment was required for forming these canisters, the high tensile residual stress existed within these canister welds. This can change the fracture resistance capacity significantly as well as increase SCC potential. Due to relative thin shell thickness of a canister weldment, the spiral notch torsion test (SNTT) method of small specimen approach was used to estimate the canister weldment fracture toughness. Most SNTT weldment samples fracture initiation sites are at heat-affected zone (HAZ) regions. The estimated fracture toughness JQ’ for the baseline SS304 steel is at 283 kJ/m2. The estimated JQ’ for the SS304/308 weld from HAZ and weld specimens are 126 kJ/m2 and 148.9 kJ/m2, respectively.

Structure and Phase Composition of Heat-Affected Zone of Austenite Steel After Deformation

The paper presents the transmission electron microscopy (TEM) investigations of the thin film structure and phase composition of the heat-affected zone (HAZ) of a weld joint produced by manual metal arc welding (MMAW) of 0.12C–18Cr–10Ni–1Ti–Fe austenite steel exposed then to plastic deformation. The test machine INSTRON-1185 is used to perform quasi-static tensile tests at room temperature and a 1.7∙10–4 s–1 strain rate up to 5 and 37% deformations. TEM investigations are carried out within the HAZ, at a 1 mm distance to the weld line, in the direction of the parent metal and at 0.5 mm distance to the weld deposit. It is shown that MMAW results in the formation of e-martensite both in the parent metal and weld deposit regions. In the latter, γ → e phase transformation occurs faster. Plastic strain ranging between 0–5% throughout the HAZ leads to further γ → e phase transformation. In the weld deposit of the HAZ region, this phase transformation is also more intensive. Further increase in the degree of plastic strain from 5 to 37% results in γ → e → α phase transformation and an elastoplastic lattice distortion of the α-phase. The plastic flexure remains in the crystal lattice of the γ-phase. The bulk material in the HAZ region satisfies the following conditions: scalar dislocation density is higher than the excess, and internal shear stresses are higher than long-ranging.

Post-weld heat treatment effects on the tensile properties of cold metal arc welded AA 6061-T6 aluminum joints

Abstract Heat treatable aluminium alloys of the 6xxx series are widely used in the hood, trunk and roof sections of a car body as they offer better corrosion resistance, high strength to weight ratio and excellent weldability characteristics. Gas metal arc welding (GMAW), a high productivity process is extensively employed in the automobile sector due to its ease of automation. HAZ softening is one of the most common problems that occurs during the welding of aluminium alloys by GMAW because of its high heat input. Cold metal transfer (CMT) is the most suitable welding process for welding aluminium alloys especially for controlling HAZ softening issues that occur due to the low heat input involved in this process. However, softening continues to exist in the welded joints. In this investigation, Post -weld heat treatment (PWHT) was employed to increase joint efficiency and to minimize the HAZ softening of AA6061-T6 aluminum alloy joints welded by GMAW and CMT. The results show that the tensile strength of a post-weld heat treated CMT joint is 10 % higher i. e., 242 MPa (76 % of joint efficiency), than that of the as welded CMT joint. This is due to the nucleation of strengthening precipitates in the weld metal and heat affected zone regions.