The Effect of Heat Treatment on Microstructures and Mechanical Properties of Flow-Formed Maraging Steel Weldment by EBW
|關鍵字:||銲接;對流旋型麻時效鋼;電子束銲接;welding;Flow-Formed Maraging Steel;EBW|
ABSTRACT The objective of this program was to study the material and fabricating process of C-250 grade 18-percent nickel maraging steel, with reference to its use in solid motor cases. Maraging steels are considered to be excellent materials for use in the aerospace industry, because of their high strength/weight ratio, superior toughness at high strength levels, good weldability, simplicity of heat treatment and ease of fabrication. The 18% nickel maraging steel has outstanding character- istics, mainly due to specific features of iron-nickel based alloys. The martensitic structure with a low rate of distortion can be attributed to the extra-low carbon (<0.03wt.﹪) content. In the solution-annealing state, after air cooling from elevated temperatures, a body-centered cubic (α-phase) structure is present and can be easily shaped using conventional process such as rolling, forging, flow forming, and others. A high-temperature aging treatment must be carried out to strengthen the maraging steel. After solution treating at 815℃ for one hour, forward flow- forming can be to produce a long and thin wall tube of C-250 maraging steel. The thickness was reduced by more than 70% in a single pass. In this research, factors which affect the precision of the C-250 tube, such as the post heat-treatment of flow-forming are studied. Under standard aging treatment condition for the formed tube, the combined effect of cold work hardening and precipitation-hardening promotes the thinning of the tube. The microhardness of the solution treated with C-250 can be increased by approximately 16.2﹪after flow forming. Direct aging is required to increase hardness and strength. Direct aging treatment yields low elongation, which is unsuited to engineering designs. The aging temperature can be increased to 540℃ under over-aged conditions to obtain a better rate of elongation of the flow-formed C-250 maraging steel tube. Given this treatment, the austenite reversion transformation can occur. With an aging treatment of 540℃/6hrs/AC, a suitable strength and rate of elongation can be obtained. The strengthening phase of the flow formed C-250 maraging steel was the intermetallic compound of Fe3Mo. However, extensive prior cold working decreases the toughness after direct aging, and the elastic modulus and toughness may be substantially anisotropy in unidirectionally worked structures. The properties of heavily cold worked structures can be improved by solution-annealing treatment. As a result, “Solution＋aging” treatment yielded greater strength and elongation. The fine equiaxed crystal（α-phase）were precipitated in the matrix and no deformed grains were observed. However, the grains of the α-phase, due to the solution- annealing of this steel, were smaller than the original grains. Under homogenization treatment, the microstructures of C-250 are of the coarse-grained lath type following aging, and of the needle type following solution＋aging. A low tensile strength can be obtained, and the tensile fracture showed a tendency to brittleness. Additionally, final formed C-250 tubes are usually manufactured by combining aging treatment and electron beam welding (EBW). The hardness and tensile strength of C-250 maraging steel declined to the corresponding values obtained in the solution-annealing state after electron beam welding. Thus, the tube formed by EBW requires stress-relieving treatment (and similarly under aging-treated conditions) to increase hardness and strength, thereby meeting required specification. After the EB welded specimen (SE2) undergoes stress-relieving treatment, the strength was basically unchanged but the elongation was 88.5% lower than necessary to meet AMS specification 6520C. Although dimples are clearly observed on the fractured surface, the result of the tensile test indicates very low ductility for the “aging＋EBW＋stress relieving” specimen. Under “formed＋EBW＋aging” condition (SE1), the tensile strength meet the required specification, and the elongation was 66% lower than required by AMS 6520C. Consequently, the mechanical properties of SE1 were batter than those of SE2, and the production process time was short. Over-aging treatment may be considered to increase the elongation to the standard value, but may reduce the tensile strength below the safe margin. Furthermore, the fusion zone and the HAZ were less hard than the base metal, by around 4 HRC. As mentioned above, the rate of elongation of the formed maraging steel weldment post heat-treatment still too low according to EBW. However, solution-aging treatment can improve the mechanical properties of heavily cold-worked structures. After solution-aging treating, the tensile strength of C-250 weldment was under the standard value（97.4%）, and the elongation was 44% lower than specified by AMS 6520C. That is, the recrystallization was completed following solution treatment of the structure in the fusion zone. However, the micro-segregation of alloy elements of the fusion zone could not be eliminated, and a large amount of austenite phase was formed in the boundary of the equiaxed grains. Then, the fusion zone was less hard than the base metal by around 2 HRC. In the tensile testing of all weldment specimens, the fracture planes developed in the coarse-grained region or along the fusion line. The mechanical properties of the formed C-250 are such that un-welded C-250 can undergo over-aging treatment. However, the preferred method involves solution-aging treatment of post-welded by EBW. Deformation due to heat treatment must be considered, as a way to adjust the process parameters of flow-forming and design fixtures for heat-treatment.