加鉬鋼材之抗硫化物應力破裂性質研究

Sulfide Stress Cracking Resistance of Molybdenum Bearing Steels

本文以0 ~ 0.45%之不同鉬（Molybdenum, Mo）含量之鋼材，探討鉬之添加對於顯微組織之改變，以及此種改變對其抗硫化物應力破裂（sulfide stress Cracking, SSC）性質之影響。鋼材之顯微組織包括肥粒鐵基地以及分散於其中的二次相組織。Mo之添加會因為壓低二次相組織的變態溫度而改變二次相之種類，由波來鐵帶逐漸變化為假性波來鐵、變韌鐵，以及麻田散鐵／沃斯田鐵島狀組織（M/A島狀組織）。SSC被發現含有兩種裂縫形態，一種平行於應力方向，而另一種則垂直之，前者可視為氫引裂之效應，而後者則為應力腐蝕破裂之貢獻。平行於應力方向之裂縫通常起始於硫化錳（Mns）與肥粒鐵基地之界面或帶狀組織上；垂直於應力方向之裂縫則主要發生在M/A島狀組織和帶狀組織上。帶狀組織和M/A島狀組織之存在均不利於鋼材之抗SSC性質，而此二因素正是影響本研究鋼材性質之最重要因素。隨著Mo含量之增加，帶狀組織逐漸消散，因此對SSC性質的有害效應漸減，至0.3% Mo時呈水平狀態；而M/A組織的有害效應則因Mo之增加而持續提高。這兩種效應的總合效果，使得含有0.3% Mo之鋼材擁有最佳的抗SSC性質。

The effects of molybdenum (Mo) content on the microstructures and sulfide stress cracking (SSC) resistance of low alloy steels were investigated. The microstructures consisted of second phases dispersed in a matrix of ferrite grains. Molybdenum additions altered the second phases from pearlite bands to psuedo pearlite, banite, and martensite/austenite (M/A) islands due to the suppression of the transformation temperatures of second phases. SSC was found to incorporate two components, one was parallel to the loading stress and frequently occurred at interfaces of MnS/ferrite and banded second phase; the other was perpendicular to the loading stress and preferentially initiated at M/A islands. The former component was related to hydrogen induced cracking and was disminished with increasing molybdenum due to the break-up of banded structure. The latter component was related to stress corrosion cracking and was enhanced by increasing molybdenum because of the formation of more M/A islands. By optimizing the above two effects, the best SSC resistance was obtained at 0.3%molybdenum.