熱處理對流旋型麻時效鋼EBW銲件顯微組織與機械性質之影響

Abstract

摘要 本論本論文主要目的係針對18﹪Ni麻時效鋼C-250運用在固體發動機燃燒室的製造程序與材料特性評估。航空工業上，麻時效鋼為一種具有高強度/重量比、優良銲接性質、簡單熱處理及易加工等優異特性的材料，而且在同等級的高強度合金鋼中之韌性表現亦非常地優越。18﹪Ni麻時效鋼是Fe-Ni為主要合金元素中重要及突出的一個金屬材料系列，因含碳量極低（＜0.03wt.﹪），在固溶-退火處理的高溫條件下，於空氣中冷卻所產生的麻田散鐵結構之晶格幾無畸變，係以體心立方結構(BCC；α相)而存在。此時之強度與硬度皆低，可以容易地以輾軋、鍛造和流旋型等加工製造技術加以成型。但是，要獲得高強度的麻田散鐵，則必須利用時效處理以析出金屬間化合物來強化材料的機械性質。 採用非切削性、無屑的順流旋型加工技術，可將固溶狀態下的麻時效鋼C-250製造成壁厚減縮率達70﹪以上之長且薄的精密高強度無縫管件。然而，因胚盂狀態下的硬度對於旋型後工件的尺寸精度非常敏感，且再加以旋型加工的大量殘留應力對於時效後所產生的壁厚減薄有加乘效果，在工件設計與結構分析模擬時就須加以考量。 旋型後的C-250硬度值僅較旋型前提高約16.2﹪，因此時效處理是需要的。然而，若直接以時效處理（480℃/6hrs/AC），則旋型C-250的拉伸強度較AMS 6520C規範值為高，延伸率則低許多，仍無法滿足工程設計上的需求。藉由540℃/6hrs/AC的過時效處理，因麻田散鐵組織產生沃斯田鐵逆變態的材質軟化反應，拉伸強度與延伸率均可以滿足規範值需求。而且，在旋型加工應力作用下之過時效處理試片中亦發現有Fe3Mo相的產生。 文獻中建議大量冷加工後，必須重新施以固溶處理，藉以恢復材料的韌性。旋型C-250在重新加以固溶-時效處理後，確實可以得到較旋型前更為細緻地等軸晶組織，而拉伸強度及延伸率亦滿足規範值的結果。然若改採以均質化熱處理條件，則C-250組織結構會受高溫作用而轉變為粗大地板條狀或針狀化結構，此時的拉伸強度會變低，而由拉伸破斷面觀察亦發現有脆性破斷的傾向。 此外，EBW銲接技術亦經常為後續加工道次中運用。但是，旋型後加以時效處理的C-250試件，若再施以EBW銲接時的銲道硬度分佈將會回復到固溶狀態下的水準，而必須再加以進行與時效條件相同的應力消除處理，藉以提高其硬度及拉伸強度至規範值。然而，此時的延伸率卻已較規範值遽降達88﹪，但由拉伸破斷面上卻非常清楚地顯示著漩渦狀延性結構。 旋型後若改採先EBW銲接，再加以時效處理，則除拉伸強度仍可滿足規範值外，延伸率之降幅已可縮減為66﹪，較先「時效＋EBW」再應力消除者為小，且製程成本亦較低。然而，因兩者之拉伸強度均與規範值相近，希藉由過時效處理方式，以犧牲強度來大幅提高延伸率將為不可行。而且，雖銲道之硬度值已達到規範值，卻仍較基材低約四個HRC刻度的差異。 由於，上述銲後熱處理條件所得到的延伸率仍然偏低，故再採以「麻時效鋼在大量冷加工後，必須再進行固溶處理」之建議。試驗結果發現：旋型C-250於EBW銲後執行固溶-時效處理者，其抗拉強度僅稍低於規範值（97.4﹪），而延伸率的降幅更是再縮減為44﹪。此種現象係由於固溶處理雖已使銲道內組織產生再結晶反應，然仍無法消除銲道內合金元素的偏析現象，反會在等軸晶粒四週產生大量地逆變態沃斯田鐵相，且其硬度分佈較基材硬度亦可減至約低兩個HRC刻度。 在所有的EBW銲件中，拉伸試驗的破斷位置均沿著銲道與熱影響區間的熔融線發展。若以機械性質考量，旋型C-250在不執行EBW銲接製程考量下，可以利用過時效處理來加以析出硬化；然而，若須加以EBW銲接時，則銲後熱處理以進行固溶-時效處理者為較佳的選擇，但其熱處理變形量將不可忽視，其解決方式或可由流旋型加工參數調整或熱處理夾具設計之變形預置或防制著手。

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.