Sheet material specimens from 18Ni (250-grade) maraging steel were gas tungsten-arc welded using two different filler wires: one matching the base material in composition and the other with higher cobalt but lower molybdenum and titanium contents. Welding was carried out both in constant-current and pulsed modes. Post-weld aging was performed at three different temperatures, viz., 425, 480 and 520°C. Metallographic characterization revealed pronounced segregation, presumably of Ti and Mo, along interdendritic and intercellular boundaries in the weld metal produced with filler of matching composition. This led, during subsequent aging, to austenite reversion at temperatures much lower than in wrought (unwelded) material. Segregation and austenite reversion were not noticed when Ti and Mo contents were reduced in the filler wire, except at the highest aging temperature used. The use of pulsed welding resulted in some grain refinement, but the effect was not considerable.

Microstructural changes during welding and subsequent heat treatment of 18Ni (250-grade) maraging steel

The frequently used aerospace materials, i.e., ultra-high-strength maraging steel (250) and corrosion-resistant 13-8 Mo stainless steel in the solution-annealed and cold-worked condition, have been joined by three fusion welding processes, namely interpulse TIG welding, and high energy density fusion processes like electron beam welding (EBW) and laser beam welding (LBW). The interpulse TIG welding process was carried out by using W2 grade maraging steel filler wire. The dissimilar joints were welded by EBW and LBW processes without any filler wire. All the dissimilar welded joints were characterized by microstructural observations and validated by mechanical properties in the as-welded as well as precipitation-hardened conditions after welding. The weld microstructures and microhardness profiles were correlated to the tensile strength of weld. Electron beam welded joint with precipitation hardening after welding, i.e., soaking at 510 °C and subsequent air cooling, demonstrated the superior mechanical properties among all the welds.

Dissimilar Welding of Maraging Steel (250) and 13-8 Mo Stainless Steel by GTCAW, LBW and EBW Processes

Microstructures of two different 18 Ni Co-free maraging specimens A and B and their electron beam (EB) weld joints were investigated comparatively by optical microscopy and scanning electron microscopy (SEM). It is shown that both of the steels are typical lath martensite; however, grain size of specimen A is about three times as large as that of specimen B. X-ray diffraction (XRD) reveals that the amount of the reverted austenitic phase in A is obviously less than that in B. Most of the austenite distributes in plate form along grain and lath boundaries while some of it distributes as fine particles within the matrix. The microstructural differences between the two specimens led to diverse behaviors in EB welding. The specimen A is weldable but B shows obvious welding defects of pits and burn-through holes in weld face. The welding microstructure exhibits a typical dendritic morphology, and the grains in the heat-affected zone recrystallized and grew because of high temperatures of welding EB. The weldablity of the examined materials is related to the microstructure characteristics that markedly affected thermal conduction performance.

Different Behavior in Electron Beam Welding of 18 Ni Co-free Maraging Steels

Electron-beam (EB) welding was used in T250 maraging steel, microstructures of both base material and heat affected zone (HAZ) were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and microhardness was tested. The results showed that during EB welding, the HAZ of T250 maraging steel exhibited a continuous gradient structure. The microstructure of the entire HAZ, from fusion line, could be divided into four zones: fusion zone, overheated zone, transition zone, and hardened zone. The microhardness showed a distinct regularity in each area. The softest region was the fusion zone, whereas the hardest was the hardened zone. In the overheated zone, the hardness increased as the grain size decreased. Furthermore, in the transition zone, the hardness level dropped noticeably. The peak temperature during the thermal cycle had a great influence on the formation of reverted austenite and dissolution of the precipitated particles, which contributed a lot to the microstructure and hardness of this material.

Microstructure and Hardness of T250 Maraging Steel in Heat Affected Zone

Fiber laser-welded joints of T-250 maraging steel with Cu layer and aging treatment were assessed with respect to tensile properties, microstructure and fractography. The hybrid process based on Cu layer and aging temperature was designed and optimized. All the welded joints fractured in the weld metal region. The approach of electroplating Cu into the weld metal was a practical option. Cu in the weld metal introduced e-Cu precipitates, leading to increase in strength, decreased reverted austenite at the grain boundaries and increased reverted austenite in the matrix, which was beneficial to toughness. High strength-high toughness welded joint was successfully obtained using the hybrid process. The strength and toughness of welded joint were simultaneously over 90% of the parent metal in the regulation window. The main controlling principle of the hybrid process was that the aging temperature (T) and Cu layer (t) meets the following criteria: f ( t ) 2 T f ( t ) 1 , where 587 °C＜T＜610 °C，17.5 µm＜t＜60 µm.

The significance and design of hybrid process in governing high strength-high toughness combination of fiber laser-welded T-250 maraging steel joint

Improving the strength of 2401 MPa maraging steel weldments is studied in this investigation. Solidified structures of weld metal are controlled by electron-beam scanning welding so that the strength of weld metal can be increased by 10%. This result is equal to that of aging treatment after welding. Simulating the electron beam isothermal heating process, local aging treatment is done on the joint and the best parameters of heat treatment are attained by means of experiment. The ultimate strength of the joint can reach 96% of the base metal's. The ultimate strength of the joint after electron beam isothermal heating local aging treatment is 93% of the base metal's. This agrees with the result of simulation. The strength of the treated joint is equal to that of solution and aging treatment after welding. The fracture appearance shows small reinforced dimples. The amount of residual austenite increases by 30%. Residual austenite uniformly distributes at the boundaries of cellular dendrites. Ti and Mo are found to segregate at the boundaries and form reinforced phases consisting of Ni and Ti or Ni and Mo, respectively. A new opinion that electron-beam high-speed isothermal heating local aging treatment strengthens austenite but not martensite to improve the strength of joints is pointed out, basing on this investigation.

Zhongku Li

Papers

A study on local heat transfer of 18Ni maraging steel after electron beam welding