大部分常用鎳鉻系奧氏體不銹鋼(gang)自高溫奧氏體狀態冷至室溫獲得的奧氏體基體組織都是亞穩定的,當繼續冷卻到室溫以下更低的溫度,或者經受冷變形,其中的一部分或大部分奧氏體會發生馬氏體轉變,變成馬氏體組織。不銹鋼中馬氏體有兩種形態:體心立方的a'馬氏體,呈鐵磁性;六方結構的ε馬氏體,為非鐵磁性。由于ε馬氏體總是與a'馬氏體伴隨而出現,有人認為ε馬氏體是γ→a'過程中的一種過渡相,也有人認為ε馬氏體是一種獨立相。


 馬氏體轉變受鋼(gang)的(de)化學成分、溫(wen)度、冷(leng)變形(xing)量及(ji)變形(xing)速(su)率(lv)的(de)影(ying)響。


  曾對不(bu)同成(cheng)分(fen)的(de)Fe-Mn-Cr-Ni合(he)金(jin)的(de)馬氏(shi)體(ti)相(xiang)(xiang)(xiang)變(bian)(bian)(bian)進行過研究(含(han)0.03%C、0.01%~0.03%Si、0.006%~0.019%N、0.012%~0.10%Al):含(han)20%Mn的(de)Fe-Mn合(he)金(jin)自(zi)1050℃水(shui)冷(leng)(leng)(leng)(leng)至(zhi)室(shi)(shi)(shi)溫(wen)(wen),發生(sheng)γ→E轉(zhuan)(zhuan)變(bian)(bian)(bian),相(xiang)(xiang)(xiang)約為(wei)(wei)(wei)(wei)(wei)40%,再(zai)冷(leng)(leng)(leng)(leng)卻至(zhi)-196℃,y相(xiang)(xiang)(xiang)幾乎全部轉(zhuan)(zhuan)變(bian)(bian)(bian)為(wei)(wei)(wei)(wei)(wei)ε相(xiang)(xiang)(xiang),只生(sheng)成(cheng)少量的(de)α'相(xiang)(xiang)(xiang)。成(cheng)分(fen)為(wei)(wei)(wei)(wei)(wei)Fe-5%Mn-15%Cr-9%Ni合(he)金(jin)自(zi)1050℃水(shui)冷(leng)(leng)(leng)(leng)至(zhi)室(shi)(shi)(shi)溫(wen)(wen),10min后(hou)(hou)冷(leng)(leng)(leng)(leng)至(zhi)-196℃,不(bu)發生(sheng)馬氏(shi)體(ti)相(xiang)(xiang)(xiang)變(bian)(bian)(bian),仍為(wei)(wei)(wei)(wei)(wei)100%的(de)y相(xiang)(xiang)(xiang);冷(leng)(leng)(leng)(leng)至(zhi)室(shi)(shi)(shi)溫(wen)(wen),再(zai)在20℃下(xia)施(shi)以40%冷(leng)(leng)(leng)(leng)變(bian)(bian)(bian)形,將(jiang)發生(sheng)γ→ε+α'的(de)轉(zhuan)(zhuan)變(bian)(bian)(bian),轉(zhuan)(zhuan)變(bian)(bian)(bian)量約為(wei)(wei)(wei)(wei)(wei)20%,其中(zhong)約2/3為(wei)(wei)(wei)(wei)(wei)ε相(xiang)(xiang)(xiang),1/3為(wei)(wei)(wei)(wei)(wei)α'相(xiang)(xiang)(xiang);如(ru)果自(zi)1050℃水(shui)冷(leng)(leng)(leng)(leng)至(zhi)室(shi)(shi)(shi)溫(wen)(wen)后(hou)(hou),再(zai)在-196℃下(xia)施(shi)以40%的(de)冷(leng)(leng)(leng)(leng)變(bian)(bian)(bian)形,發生(sheng)γ→e+α'的(de)轉(zhuan)(zhuan)變(bian)(bian)(bian),轉(zhuan)(zhuan)變(bian)(bian)(bian)量近90%,其中(zhong)約1/4為(wei)(wei)(wei)(wei)(wei)ε相(xiang)(xiang)(xiang),3/4為(wei)(wei)(wei)(wei)(wei)a'相(xiang)(xiang)(xiang)。成(cheng)分(fen)為(wei)(wei)(wei)(wei)(wei)Fe-20%Cr-12%Ni的(de)合(he)金(jin)自(zi)1050℃水(shui)冷(leng)(leng)(leng)(leng),10min后(hou)(hou)冷(leng)(leng)(leng)(leng)至(zhi)-196℃,不(bu)發生(sheng)馬氏(shi)體(ti)相(xiang)(xiang)(xiang)變(bian)(bian)(bian),冷(leng)(leng)(leng)(leng)至(zhi)室(shi)(shi)(shi)溫(wen)(wen)在20℃下(xia)施(shi)以40%的(de)冷(leng)(leng)(leng)(leng)變(bian)(bian)(bian)形,僅發生(sheng)γ→E轉(zhuan)(zhuan)變(bian)(bian)(bian),相(xiang)(xiang)(xiang)約為(wei)(wei)(wei)(wei)(wei)10%;如(ru)果冷(leng)(leng)(leng)(leng)至(zhi)室(shi)(shi)(shi)溫(wen)(wen)后(hou)(hou)再(zai)在-196℃下(xia)施(shi)以40%的(de)冷(leng)(leng)(leng)(leng)變(bian)(bian)(bian)形,將(jiang)發生(sheng)γ→ε+α轉(zhuan)(zhuan)變(bian)(bian)(bian),轉(zhuan)(zhuan)變(bian)(bian)(bian)量約為(wei)(wei)(wei)(wei)(wei)55%,15%為(wei)(wei)(wei)(wei)(wei)ε相(xiang)(xiang)(xiang),40%為(wei)(wei)(wei)(wei)(wei)α'相(xiang)(xiang)(xiang)。


對于奧氏體不銹鋼的(de)(de)馬氏體轉變的(de)(de)臨界溫度M3,已(yi)建立起(qi)若干Ms,點(a')與合金成分關系的(de)(de)經驗公式,下面是其中的(de)(de)一(yi)個表達式:


式 10.jpg


  上面(mian)兩個公式(shi)說明,奧(ao)氏體不(bu)銹鋼(gang)中(zhong)合金元素含量(liang)越(yue)(yue)高,馬(ma)氏體點(dian)就越(yue)(yue)低,馬(ma)氏體轉變(bian)就越(yue)(yue)不(bu)容易(yi)發生(sheng)。


   在18Cr-8Ni奧氏體不銹鋼中,α'馬氏體形成量隨冷變形量加大和變形溫度降低而增多,如圖9.46和圖9.47所示。可以看出,在每一變形溫度下,α'馬氏體含量隨冷變形量加大而增長到一定數值后將趨于飽和。隨變形溫度的降低,在同樣變形量的條件下,α'馬氏體的生成量增加,其增加也有一個飽和值,亦即無論采取什么措施,不可能完全轉變為馬氏體。這種奧氏體穩定化受多種因素影響,除合金成分和晶粒度等內在因素外,還有奧氏體化溫度、冷卻速率等外界因素。


46.jpg


 變形(xing)速率對馬氏體轉變的(de)(de)影響(xiang)主要是變形(xing)發熱導致材料溫度(du)上(shang)升的(de)(de)作用(yong),即變形(xing)速率越高(gao),材料溫度(du)也(ye)越高(gao),因而α'馬氏體的(de)(de)生成量也(ye)越少。