Maraging Steel Weldments Research Articles

A novel approach to suppression of reverted austenite in aerospace grade thick maraging steel weldments using the turbo-TIG welding process

This research examines aerospace grade-maraging steel weldments produced by the novel Turbo-TIG welding technique. The vital challenge in welding maraging steel is the development of reverted austenite in the weld region and HAZ. This reverted austenite degrades the mechanical characteristics of weldments. MDN 250 maraging steel with a 12 mm thick plate was welded at a high speed of 300 mm/min with consistent quality using the Turbo-TIG welding technique. The objective of the present work is to suppress the reverted austenite in the 12 mm thick plate of MDN 250 using the Turbo-TIG welding technique. Three different postweld heat treatments were applied, namely, (i) aging heat treatment (AT), (ii) solutionizing + aging (ST), and (iii) homogenization + solutionizing + aging (HT). The microstructure and X-ray diffraction analysis of the weld zone under AT and ST conditions demonstrate the existence of reverted austenite in the grain boundary regions. The SEM/EDS analysis of the weld zone of welded, AT, and ST conditions indicated Ni, Mo, and Ti microsegregation from the center matrix to the grain boundary regions. In contrast, neither microsegregation nor austenite reversion occurred in the HT condition. The tensile test demonstrates a maximum strength of 1797 MPa (UTS) under HT conditions. Likewise, the microhardness of the HT conditions exhibits a higher hardness of 552 HV in the fusion zone. In light of the metallurgical and mechanical characteristics of the weld, the HT heat treatments provide better weld performance in the case of Turbo-TIG welding of maraging steel grade MDN 250.

Influence of microstructure on near-threshold fatigue crack growth behavior of 18Ni maraging steel weldment

Influence of microstructure on near-threshold fatigue crack growth behavior of 18Ni maraging steel weldment

Development of a Novel Welding Technique With Reduced Heat-Input by Employing Double-Pulsed Gas Metal Arc Welding for Aerospace Grade 18% Ni Maraging Steel

Abstract Eighteen percent Ni maraging steels are high performance Fe–Ni martensite-based alloys with ultra-high strength and good toughness. They find applications in strategic sectors, joining of thick sections often coming into picture. Welding of thick section involves a longer processing time, more passes, and a higher heat-input. Double-pulsed gas metal arc welding (DP-GMAW) is an emerging welding technique, well suited for joining thick sections. DP-GMAW is capable of controlling the solidification parameters, weld pool geometry, and cooling rate at a reduced heat-input. The major concern regarding the welding of maraging steel is the formation of the reverted austenite (RA) phase in the fusion zone (FZ). The formation of RA deteriorates the mechanical performance of welded joints. The presence of RA can be supressed by the usage of suitable welding techniques and proper post-weld heat treatments (PWHTs). DP-GMAW process was employed to carry out the welding; studies on the joints produced are reported in this research paper. The studies also included the effect of various PWHTs on the metallurgical and mechanical properties of the maraging steel weldments. The research used three distinct PWHTs: direct aging (DA), solutionizing + aging (SA), and homogenizing + solutionizing + aging (HSA). The FZ microstructures under DA and SA conditions show that there is RA at the cell boundaries. However, there was no evidence of RA in FZ following HSA. The energy dispersive spectra (EDS) analysis of the as-welded FZ showed segregation along the grain boundaries (GBs). This led to the premature formation of RA upon subsequent aging. The SA treatments proved inadequate to totally eliminate RA in the microstructure. On the other hand, the HSA treatments were effective in evening out concentration differences and preventing formation of RA. This study demonstrates that DP-GMAW combined with HSA treatment has the best mechanical properties.

Effect of tool material and process parameters on friction stir weld formation of maraging steel

ABSTRACT 18% Ni maraging steel is an ultra-high strength steel widely used in defence and aerospace applications. Fusion welding of maraging steel is encountered with a drop in properties due to the segregation of alloying element and hence, solid-state welding process such as friction stir welding (FSW) is a possible alternative. However, welding of high strength material using FSW is a challenge considering the tool material durability, tool material cost, and selection of tool geometry and process parameters. Therefore, the main focus of this paper is towards the selection of tool geometry, tool material, and process parameters that will result in least tool wear/failure in addition to obtaining defect-free welds of maraging steel using FSW. It was observed that polycrystalline cubic boron nitride (PCBN) and tungsten-molybdenum are durable tool material for welding maraging steel. It is recommended to use a minimum tool pin diameter (top 12 mm and bottom 8 mm) and shoulder diameter (20 mm) below which failure of pin and surface defects are encountered during FSW. It is also observed that there exists a narrow range of tool rotational speed (250–450 rpm) and tool travel speed (25 mm/min) at which defect-free maraging steel welds can be obtained.

An investigation into microstructure and mechanical properties of maraging steel weldment

Maraging steel is an alloy of Ni-Fe having very low carbon content. These steel is having superior properties like toughness and strength without losing its malleability. In the present work 5mm thick Maraging steel plates were welded using GTAW process with maraging steel filler wire. This paper attempts to analyse the microstructure and mechanical properties of the weldment. Post weld heat treatment (PWHT) was carried out at 480°C. Strength of welded joint of as-weld and PWHT was tested using ultimate tensile machine (UTM). Both the tensile specimen (As-weld and PWHT) breaks at base metal zone. The quality of joint at micro-level was analysed using optical microscopy. PWHT sample have finer grain compare to weld sample. Different phases transformation was observed in microstructures. Microhardness was also observed at different location in and around the joint. It was also observed that the fusion zone and heat affect zone (HAZ) of PWHT has higher value of Microhardness compare to weld sample.

Environmentally Assisted Cracking Behavior of 18Ni 250 Maraging Steel Welds Obtained with and Without Flux in Chloride Media

Gas tungsten arc welding is extensively used in the fabrication of 18Ni 250 maraging steel motor casings. In this work, the role of flux addition on the environmentally assisted cracking (EAC) and electrochemical corrosion behaviors of these welds in their peak-aged condition is presented. Studies were performed in order to bring out the potential influence of long-term exposure of maraging steel welds to neutral 0.1 M NaCl (mildly corrosive) and acidified 0.1 M NaCl (a relatively more corrosive) environments to understand the EAC behavior of the welds. The study showed that both the fluxed and unfluxed weld metals suffered 21% to 23% loss in ductility in neutral 0.1 M NaCl solution from that of obtained in air. The EAC of the welds in the neutral chloride solution was due to anodic dissolution, while in the acidified chloride solution it was found to be predominantly due to hydrogen embrittlement. The effect of flux in refining the weld microstructure was reflected in: (a) preventing intergranular mode of hydrogen embrittlement of the unfluxed welds and transforming the mode to transgranular cracking, (b) lowering a wide scatter in electrochemical corrosion rate, and (c) reducing the selective attack in the interdendritic regions of the unfluxed welds. Such a transition from intergranular to transgranular cracking might affect long-term service behavior of the welds to corrosion and EAC, though it is not reflected in the short-term tests.

Environmentally assisted cracking susceptibility of dark etched HAZ/ parent metal interface region of 18Ni 250 maraging steel weldment

Gas tungsten arc welding is widely employed to fabricate large sized motor casings made of 18Ni 250 maraging steel. However, small transverse cracks have been found to appear at the interface of the dark etched heat affected zone (HAZ) of such weldments after about a year of welding, prior to aging treatment. In order to understand the delayed cracking phenomena vis-à-vis the influence of multiple weld passes, thermal cycles were obtained at different locations of the heat affected zone. The results suggest the formation of two more HAZs designated as HAZ III and HAZ IV in addition to those two currently reported in the literature. The two regions namely, HAZ III and HAZ IV, respectively correspond to peak aging and under aging conditions of the alloy. In order to examine the effect of these thermal cycles on the environmentally assisted cracking of the weldment, thermal simulations corresponding to various HAZs were applied on to specimens with Gleeble. They were examined for their susceptibility to EAC in neutral and acidified 0.1 M NaCl solutions. The study reveals that it is only the HAZ IV, lying at the interface of the dark etched HAZ, corresponding to under aging condition, is responsible for the weld failure.

A critical review on susceptibility of stress corrosion cracking in maraging steel weldments

Stress corrosion cracking is the decay of mechanical characteristics of sensitive material under the mixed effects of stress and corrosive atmosphere. Because of their high strength and extremely good fracture toughness 18 Ni maraging steel (250 grade) is suitable for aerospace application such as rocket engines as these steel materials can be welded without much hassles over a broad range of thickness. The maraging steel weldment causes catastrophic failure during service owing to stress corrosion cracking. It is a challenge to comprehend the SCC behaviour for the weldment of this material. The attempts of this review paper are to analyse the mechanical and microstructural changes related to SCC due to some post weld heat treatment or surface modifications. Further, review was made to intuit the effect of different welding processes on SCC behaviour of this material. Apart from a brief review of 18 Ni maraging steel, this paper presents the impact of different corrosive environments on SCC activity. The effect of different filler materials and composition of the filler material were also investigated.

Effect of Cr Contents and Heat Treating on Reverted Austenite in Maraging Steel Weldments

By conducting flux cored arc welding (FCAW) on maraging steels with Cr contents of 1.4 and 5.2 wt%, this study observed the effects of Cr content and heat treating on reverted austenite formation in welded maraging steel. Aging treatment was carried out at the temperatures of 450, 480 and 530 °C for 3 h in each condition. As the aging temperature increased, reverted austenite was formed along the interdendritic and intercellular grain boundaries, and the proportion of reverted austenite increased with increasing Cr addition. The aging process led to the segregation of Ti and Mo along the interdendritic and intercellular grain boundaries. Some of the welded specimens were subjected to solution heat treatment at 820 and 1250 °C for 1 h after welding, resulting in a decrease in reverted austenite fraction.

Analysis of Slag Formation in Maraging Steel Weldments

18%Ni maraging steel (M250) is used in the form of welded hardware in launch vehicles. GTAW process using W2 filler wire is being followed to realise this hardware. As compared to M250, W2 filler has a lower amount of Ti and Mo. Al content is increased to 0.4 wt.% to impart high fracture toughness to the weldment. In recent past, manufacturing industries are experiencing a severe problem of slag formation in M250 weldments. Slag can be removed only by extensive inter-pass grinding. This type of slag formation in TIG welding is an unusual phenomenon and has not been reported for maraging steels. This work attempts to understand the root cause of slag formation. Extensive welding trials have been carried out on M250 steel using W2 filler wire to simulate the slag formation. The slag is hard and adherent to the weldment and it cannot be removed by simple wire brushing. Detailed analyses of slag, M250 base metal, and filler wire were carried out using SEM-EDS and XRD. Slag is found to be a variant of calcium aluminate. The main constituents are alumina (Al2O3) and calcium oxide (CaO). Based on the above observation, different contributing factors for the formation of slag and their role in the severity of slag formation are discussed. Various possible solutions to overcome this problem are also addressed.

Role of Process Parameters on Bead geometry and Metallurgical Characteristics in autogenous Gas Tungsten Arc Welding of Maraging Steels (250 grade)

This research aims to characterize the bead geometry, metallurgical aspects of the optimized maraging steel (250 grade) weld track processed by autogenous gas tungsten arc welding technique. Design of experiments was used to determine the weld track possessing optimal bead geometry. Welding speed and current were the parameters considered for optimization. Depth of penetration and bead width were the adjudicating parameters for optimality. Taguchi analysis showed that welding current is directly proportional to depth and width whereas weld speed was indirectly proportional. Statistical analysis indicated that welding speed has affected the depth of penetration and different current levels have affected the width of the weld bead linearly. Optical microscopy and SEM/EDAX analysis were done to investigate the post weld microstructural changes. X-Ray Diffraction were done to understand the phase formations.

Stress corrosion cracking behaviour of gas tungsten arc and friction stir maraging steel welds

Maraging steel, an ultra high strength steel having high alloying elements and low carbon, finds its application in many critical components including military and aerospace. As the strength of a material increases its susceptibility to stress corrosion cracking (SCC) also increases. A study is carried out to understand the SCC behaviour of MDN-250, welded through gas tungsten arc welding (GTAW) and friction stir welding (FSW) process. Melting and re-solidification during GTAW of maraging steel results in formation of reverted austenite at prior austenite grain boundaries and dendrite boundaries, in the form of pool, due to alloying elements segregation. The interface of austenite pool and surrounding martensite matrix is the corrosion susceptible region. GTAW (fusion welding) process results in coarsening of grains at the weld and HAZ which is more prone to SCC. There is no segregation of the alloying elements; formation of reverted austenite is thus restricted in FSW. A very high strain rate deformation achieved through FSW results in very fine grain structure which cannot be achieved through any other welding process. Better SCC properties of FS-welded maraging steel can be attributed to the fine grain structure and absence of reverted austenite in form of pool.

Influence of Welding Techniques on Heat Affected Zone Softening of Dissimilar Metal Maraging Steel and High Strength Low Alloy Steel Gas Tungsten Arc Weldments

The present investigation reports on a study that has been taken up to understand and control the extent of softening in heat affected zone of dissimilar gas tungsten arc (GTA) welds of high strength steels: AISI 4130 steel and 18 % Ni maraging steel which are in hardened condition i.e., maraging steel in solution treated and aged condition and AISI 4130 steel in hardened and tempered condition before welding. The influence of welding technique on the width of soft zone and in-turn on the hardness has been investigated. All the dissimilar joints have been investigated for microstructures, microhardness and tensile properties. It is observed that continuous current GTA welding process results in a wider soft zone, leading to poor mechanical properties, which can be attributed to softer microstructural constituents.

Prediction of Temperature Profile in the HAZ of Maraging Steel GTAW Weldments

This paper discusses the modeling of gas-tungsten arc welding (GTAW) process being adopted for maraging steel plates used in aerospace applications. Primary objective of this work is to predict the temperature profile of heat affected zone in a maraging steel weld. Finite element modeling was done utilizing a gaussian heat flux distribution on a weldment. Model also incorporated a moving heat source along with provision for heat sinks in the form of copper backup plates. For validating the model, welding experiments were conducted on 2 mm and 8 mm thick 250 grade maraging steel plates. Thermal profiles were acquired at different locations of heat affected zone using thermocouples and compared with the predictions from the model.

Role of pulsed current on metallurgical and mechanical properties of dissimilar metal gas tungsten arc welding of maraging steel to low alloy steel

This research work encompasses the investigations carried out on the mechanical and metallurgical properties of maraging steel and AISI 4340 aeronautical steel weldments. The materials were joined by continuous current gas tungsten arc welding (CCGTA) and pulse current (PCGTA) gas tungsten arc welding processes using ErNiCrMo-3 filler wire. Cross sectional macrostructures confirmed proper deposition of the fillers and lack of discontinuities. Optical microscopy studies revealed that at the maraging steel–weld interface, martensite in distorted and block forms prevailed in CCGTA and PCGTA weldments whereas tempered martensite was predominant at the low alloy–weld interfaces of both the welds. Scanning electron microscopy (SEM) with energy dispersive analysis of X-rays (EDAX) analysis apparently showed less elemental migration in PCGTA weldments as compared to the other. Results of X-ray diffraction analysis recorded possible phase formations in various zones of the weldments. Microhardness profiles in either weld zones followed a constant trend whereas it showed a downtrend in the heat affected zones (HAZ) of the maraging steel and very high hardness profiles were observed in the low alloy steel side. Tensile studies on various factors and impact testing showed that PCGTA weldments outperformed the continuous ones in terms of strength, ductility and toughness. Fractograph analysis depicted the nature of failures of tensile and impact tested specimens. Comparison analyses involving influence and nature of pulsed current welds over continuous ones were done to determine the possibility of implementing these joining processes in aerospace applications. Weldments fabricated using PCGTA technique proved to be superior to the other, resulting in exceptional mechanical properties.

Studies on Welding of Maraging Steels

This work involves with investigations on Maraging steel weldments done by GTA welding technique. Similar welding of maraging steel plates of 5mm thick using ErNiCrMo-3 filler wire was done in order to evaluate the weldability and performance characteristics. Micro structural and X- Ray diffraction analysis were done to analyze the structure and phases developed post – weld. Block martensite was formed in the heat affected zones of the parent metal. Tension testing using different strain rates and hardness profile measurements were done to investigate the mechanical properties and the study concludes that the strain rate has considerable influence on the nature of ductility whereas it has very less influence on the Ultimate Strength of Maraging steel.

Key- Hole Plasma Arc Welding Of 8 Mm Thick Maraging Steel — A Comparison With Multi- Pass Gtaw

Maraging steel is an iron nickel alloy with typical 18Ni-8Co-5Mo composition. It has low C martensitic structure which is strengthened by precipitation of inter-metallic compounds through ageing. Due to good fracture toughness in addition to high strength to weight ratio, the steel is widely used in aerospace applications. Conventionally we have been welding 8 mm-thick maraging steel with multi-pass automatic GTAW process. The steel shows “austenite reversion” phenomenon when heated to two phase (austenite + ferrite) region. Due to this phenomenon, maraging steel weld has two peculiar heat-affected zones; zone A nearest to the weld which is heated to fully austenitic region and zone B adjacent to it is heated to two phase region. Zone B contains reverted austenite which is not hardened by ageing. In conventional GTAW weld, zone B is quite wide, due to cumulative effect of multipasses. As zone B is not hardened fully by aging, it affects the mechanical properties of weldments. To reduce the width of the HAZ (zone B), single pass key-hole Plasma Arc Welding (PAW) was evaluated for 8 mm-thick plate. This paper describes the effect of weld passes and heat input on weld properties as a comparative study of single pass PAW and multi-pass GTAW. Mechanical properties (hardness, tensile properties, and fracture toughness) and macroscopic observations have been discussed. Also filler wire consumption & arcing time has being compared which are important aspects for production. The effect of single pass from multi-pass is very much visible in macroscopic observations. PAW weld shows significant reduction in width of HAZ (zone B) as compared to multi-pass GTAW. In aged condition in PAW weld does not show sudden drop in hardness in HAZ which is a common phenomenal in GTAW & the Improvement has been observed in tensile properties of PAW weld.

Improvement of mechanical properties of stainless maraging steel laser weldments by post-weld ageing treatments

The response of stainless maraging steel weldments to post-weld ageing treatment has been investigated. Post-weld ageing was performed at five different temperatures, viz., 420°C, 460°C, 500°C, 540°C, and 580°C. Metallographic characterization of weldment revealed three zones, namely fusion zone, heat-affected zone (HAZ) and unaffected parent metal zone. Hardness and tensile properties were evaluated after ageing at different temperatures. Hardness in HAZ and fusion zone varied with ageing temperature differently from that of the parent metal; it became higher in HAZ and fusion zone than in parent metal zone above 420°C. Among the applied ageing treatments, ageing at 460°C achieved the highest tensile strength. A graph was constructed for determination of fracture location and post-weld heat treatment efficiency based on experimental results, using hardness ratio of HAZ to the treated parent material and hardness ratio of HAZ to the as-received parent material.

Dissimilar Metal Gas Tungsten Arc Weldments of Maraging Steel and Medium Alloy Medium Carbon Steel – Effect of Post-Weld Heat Treatments

Maraging steel and medium alloy medium carbon steels exhibit their best mechanical properties such as tensile strength and toughness in their respective heat treatment conditions. Gas tungsten arc welding of maraging steel and medium alloy medium carbon steel was carried out taking both the steels in soft annealed condition. Later the weldments were subjected independently to two post-weld heat treatments, one corresponding to the maraging steel i.e. solutionising at 815°C/1 hr/air cooled&aging at 480°C/3 hrs/air cooled, and the other corresponding to medium alloy medium carbon steel i.e. quenching at 925°C/35 min/air cooled&tempering at 295°C/45 min/air cooled. The effect of post-weld heat treatments on the microstructure and mechanical properties such as hardness, tensile strength and impact toughness of the dissimilar metal welds of maraging steel and medium alloy medium carbon steel was investigated. The influence of filler materials was also studied by employing maraging steel and medium alloy medium carbon steel fillers. Maraging steel welds responded to the solutionising and aging treatment whereas the medium alloy medium carbon steel welds responded to quenching and tempering. Lowering of the hardness was observed at the interaction of maraging steel and medium alloy medium carbon steel due to the diffusion of manganese. Medium alloy medium carbon steel filler welds showed good strength and toughness properties.

Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments

The response to post-weld heat treatment of an 18%Ni (250 grade) gas tungsten arc weld metal has been investigated. The post-weld heat treatments are (a) direct aging at 480°C/3 h/air cooling, (b) solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling and (c) homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling. Metallographic characterisation of fusion zone revealed pronounced segregation of titanium and molybdenum along the interdendritic and intercellular boundaries. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Solutionised treatment at 815°C does not remove the segregation. Homogenisation treatment (1150°C/1 h/air cooling) succeeded in making the composition become homogenised. Mechanical properties including tensile, hardness and impact toughness were evaluated. Tensile test results showed that directly aged weldments exhibited lower strength but higher ductility than the other cases; this was attributed to the presence of reverted austenite. Homogenisation at 1150°C/1 h/air cooling+solutionising at 815°C/1 h/air cooling+aging at 480°C/3 h/air cooling resulted in optimum tensile properties. A substantial increase in fusion zone toughness was observed after homogenisation+solutionising+aged condition due to a decrease in the content of austenite content compared to the directly aged condition. The reduction in microsegregation by diffusion of alloying elements from cell boundaries to the cell during homogenisation treatment is responsible for the decrease in austenite content.