奧氏體不銹鋼具有良好的耐蝕性和抗氧化性,并具有高的塑性和良好的焊接性。奧氏體不銹鋼在航空、汽車、造船、化工、醫療器械以及人民日常生活中得到了廣泛的應用,并且由于這類鋼無磁性,因此在儀表、儀器工業中應用得也很廣泛。
但是(shi)奧氏體不(bu)(bu)(bu)銹鋼在加(jia)(jia)(jia)熱(re)和(he)冷卻時(shi)不(bu)(bu)(bu)發生(sheng)α→y固態相變(bian),因(yin)此(ci)不(bu)(bu)(bu)能通過熱(re)處理來達到強(qiang)化的(de)目的(de),只能通過冷變(bian)形加(jia)(jia)(jia)工來提(ti)高鋼的(de)強(qiang)度(du)(du)(du)。但是(shi)這種提(ti)高強(qiang)度(du)(du)(du)的(de)方法,只能用于工作(zuo)溫(wen)度(du)(du)(du)不(bu)(bu)(bu)高的(de)零件,否則其強(qiang)度(du)(du)(du)將因(yin)高溫(wen)加(jia)(jia)(jia)熱(re)而迅(xun)速降低。
奧(ao)氏體不(bu)銹(xiu)鋼(gang)的熱處理(li)方(fang)式(shi)有(you)以下(xia)三種(zhong):
1. 去(qu)應力退火
為了(le)消除加工后的(de)殘留應力,可加熱到300~350℃,保溫1~2h,空冷。
消(xiao)除(chu)焊接(jie)應力時一般采用700~900℃退火(huo),保溫(wen)1~3h,空冷。溫(wen)度越(yue)高,焊接(jie)應力消(xiao)除(chu)得越(yue)迅速越(yue)徹底,而其軟化效果也越(yue)大。
用來消除(chu)冷作(zuo)硬化的退(tui)火,其溫度為750~850℃,保溫時間為1~3h,快冷。
2. 固溶處理
為使(shi)奧(ao)氏(shi)體不(bu)銹鋼(gang)中(zhong)的(de)(de)碳化物及其(qi)他化合物(σ相分解物)充分溶(rong)入(ru)奧(ao)氏(shi)體,然后快速(su)冷(leng)卻到室(shi)溫,以獲得單相奧(ao)氏(shi)體的(de)(de)工藝(yi)方(fang)法(fa)叫固溶(rong)處理。其(qi)工藝(yi)與(yu)淬火工藝(yi)相似,只是(shi)鋼(gang)中(zhong)不(bu)發生(sheng)相變(bian),處理的(de)(de)室(shi)溫組織是(shi)過飽和的(de)(de)y-Fe固溶(rong)體。固溶(rong)處理的(de)(de)目的(de)(de)是(shi)提高(gao)奧(ao)氏(shi)體型不(bu)銹鋼(gang)的(de)(de)耐蝕(shi)性和抗高(gao)溫氧(yang)化性,也(ye)可以消除冷(leng)作硬化。
固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)處理的(de)(de)加(jia)熱(re)(re)(re)溫(wen)(wen)度(du)(du)都(dou)較(jiao)高(gao)(gao),一般(ban)為(wei)(wei)(wei)(wei)1000~1100℃,含碳(tan)(tan)量(liang)高(gao)(gao)時(shi)取上限(xian)溫(wen)(wen)度(du)(du),含碳(tan)(tan)量(liang)低時(shi)取下限(xian)溫(wen)(wen)度(du)(du)。奧(ao)氏(shi)體型(xing)不(bu)(bu)銹(xiu)(xiu)(xiu)鋼應(ying)(ying)在(zai)中(zhong)(zhong)性或弱氧化(hua)氣氛中(zhong)(zhong)加(jia)熱(re)(re)(re),為(wei)(wei)(wei)(wei)此(ci)(ci)常采(cai)用(yong)空(kong)氣爐(lu)為(wei)(wei)(wei)(wei)加(jia)熱(re)(re)(re)設備并以氨分解氣等作(zuo)為(wei)(wei)(wei)(wei)加(jia)熱(re)(re)(re)介質。氯化(hua)鹽會(hui)使(shi)鋼遭受腐(fu)蝕(shi),故不(bu)(bu)宜用(yong)鹽浴(yu)加(jia)熱(re)(re)(re),在(zai)有(you)條件(jian)的(de)(de)地方最好(hao)采(cai)用(yong)真空(kong)爐(lu)加(jia)熱(re)(re)(re)。在(zai)加(jia)熱(re)(re)(re)奧(ao)氏(shi)體不(bu)(bu)銹(xiu)(xiu)(xiu)鋼時(shi),既應(ying)(ying)阻止(zhi)(zhi)(zhi)鋼的(de)(de)表(biao)面氧化(hua),也應(ying)(ying)防(fang)(fang)止(zhi)(zhi)(zhi)鋼的(de)(de)表(biao)面增(zeng)碳(tan)(tan),因(yin)為(wei)(wei)(wei)(wei)增(zeng)碳(tan)(tan)的(de)(de)結果會(hui)使(shi)鋼的(de)(de)晶(jing)間(jian)(jian)腐(fu)蝕(shi)傾向(xiang)增(zeng)大(da)。一般(ban)來說,加(jia)熱(re)(re)(re)溫(wen)(wen)度(du)(du)越高(gao)(gao),碳(tan)(tan)化(hua)物的(de)(de)固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)速(su)度(du)(du)越快,如06Cr9Ni10鋼的(de)(de)碳(tan)(tan)化(hua)物在(zai)1000℃固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)需要10min,在(zai)1065℃固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)需要3min,在(zai)1176℃固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)需要1.5min。從(cong)這(zhe)個(ge)角度(du)(du)來講,加(jia)熱(re)(re)(re)溫(wen)(wen)度(du)(du)越高(gao)(gao)越好(hao),但加(jia)熱(re)(re)(re)溫(wen)(wen)度(du)(du)偏高(gao)(gao)又會(hui)引起晶(jing)粒過分長(chang)(chang)大(da),氧化(hua)鐵皮(pi)增(zeng)厚(hou)。因(yin)為(wei)(wei)(wei)(wei)奧(ao)氏(shi)體不(bu)(bu)銹(xiu)(xiu)(xiu)鋼無法通過相變(bian)來細化(hua)晶(jing)粒,如果晶(jing)粒過大(da)會(hui)使(shi)材(cai)料的(de)(de)抗拉強度(du)(du)明(ming)顯(xian)下降。為(wei)(wei)(wei)(wei)保(bao)證加(jia)熱(re)(re)(re)質量(liang),固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)處理前必須將奧(ao)氏(shi)體不(bu)(bu)銹(xiu)(xiu)(xiu)鋼零件(jian)表(biao)面清洗干(gan)凈。奧(ao)氏(shi)體不(bu)(bu)銹(xiu)(xiu)(xiu)鋼在(zai)低溫(wen)(wen)時(shi)的(de)(de)熱(re)(re)(re)導率較(jiao)低,升到高(gao)(gao)溫(wen)(wen)后(hou)(700~800℃)熱(re)(re)(re)導率才有(you)所提高(gao)(gao),因(yin)此(ci)(ci)奧(ao)氏(shi)體不(bu)(bu)銹(xiu)(xiu)(xiu)鋼零件(jian)在(zai)低溫(wen)(wen)加(jia)熱(re)(re)(re)時(shi),為(wei)(wei)(wei)(wei)保(bao)持(chi)零件(jian)內外(wai)均衡加(jia)熱(re)(re)(re),加(jia)熱(re)(re)(re)速(su)度(du)(du)要緩慢,對于(yu)斷面較(jiao)大(da)的(de)(de)奧(ao)氏(shi)體不(bu)(bu)銹(xiu)(xiu)(xiu)鋼零件(jian)要預熱(re)(re)(re)到700~800℃等溫(wen)(wen)到一定時(shi)間(jian)(jian),再快速(su)升溫(wen)(wen)。在(zai)固(gu)(gu)溶(rong)(rong)(rong)(rong)(rong)溫(wen)(wen)度(du)(du)下的(de)(de)保(bao)溫(wen)(wen)時(shi)間(jian)(jian)不(bu)(bu)宜過長(chang)(chang),一般(ban)按材(cai)料的(de)(de)有(you)效厚(hou)度(du)(du)計算,一般(ban)每毫米保(bao)溫(wen)(wen)1min。為(wei)(wei)(wei)(wei)防(fang)(fang)止(zhi)(zhi)(zhi)碳(tan)(tan)化(hua)物從(cong)奧(ao)氏(shi)體中(zhong)(zhong)析(xi)出(chu),加(jia)熱(re)(re)(re)保(bao)溫(wen)(wen)后(hou)應(ying)(ying)在(zai)水中(zhong)(zhong)快速(su)冷卻,對于(yu)厚(hou)度(du)(du)不(bu)(bu)大(da)的(de)(de)工件(jian),為(wei)(wei)(wei)(wei)防(fang)(fang)止(zhi)(zhi)(zhi)變(bian)形,可以空(kong)冷。
3. 穩(wen)定化處理
對(dui)于含鈦、鈮的穩定(ding)型鉻、鎳奧氏(shi)體(ti)不銹鋼,可以采用穩定(ding)化(hua)處理(li),以提高(gao)鋼在(zai)(zai)450~900℃溫度(du)范圍內、并在(zai)(zai)強(qiang)烈腐蝕介(jie)質中(zhong)(zhong)工作時的耐蝕性。穩定(ding)化(hua)處理(li)是在(zai)(zai)固溶處理(li)以后進行,穩定(ding)化(hua)處理(li)工藝是將(jiang)鋼加(jia)熱到850~900℃,保溫2~4h,然后在(zai)(zai)水中(zhong)(zhong)或(huo)空氣中(zhong)(zhong)冷卻(que)。
這是因為鋼中加入鈦和鈮可消除晶間腐蝕,但它們的效果必須通過穩定化處理才能得到保證。單純的固溶處理使鉻的碳化物全部溶入奧氏體中,鈦和鈮的碳化物則部分溶入奧氏體中,如經400~800℃加熱,溶入奧氏體中的碳大部分以Cr23C6的形式析出,鈦和鈮的固定作用不能有效地發揮,晶間腐蝕仍會發生。如果將穩定化溫度提高到高于Cr23C6的溶解溫度,且又是碳化鈦(或碳化鈮)強烈析出的溫度(850~900℃),保溫2~4h,就能夠形成穩定的碳化鈦(或碳化鈮),從而防止奧氏體不銹鋼的晶間腐蝕。