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船舶柴油發(fā)動機(jī)氣缸蓋裂紋原因與維修

2012-8-6 14:47:25

　柴油機(jī)氣缸蓋不僅結(jié)構(gòu)復(fù)雜，而且工作條件、受力情況也十分復(fù)雜、惡劣。氣缸蓋的底面直接與高溫高壓燃?xì)饨佑|，不僅受到高溫高壓燃?xì)獾母g與沖刷，而且周期地承受較高的機(jī)械負(fù)荷與熱負(fù)荷；底面在燃?xì)獾谋l(fā)壓力作用下產(chǎn)生很大的機(jī)械應(yīng)力，壁厚與溫度的不均勻?qū)е螺^高的熱應(yīng)力；螺栓的預(yù)緊力使氣缸蓋承受著壓應(yīng)力，并與燃?xì)鈮毫餐饔檬箽飧咨w受到彎曲作用，在截面變化處會產(chǎn)生應(yīng)力集中等。此外，冷卻水腔還受到水的腐蝕。

　　正是由于氣缸蓋如此惡劣的工作條件，致使氣缸蓋很容易損壞。通常損壞形式為：其底面和冷卻水腔容易產(chǎn)生裂紋，還有氣閥底面和導(dǎo)套容易磨損，冷卻水側(cè)被腐蝕等。

本文主要針對船舶柴油機(jī)氣缸蓋最經(jīng)常出現(xiàn)的損壞現(xiàn)象——裂紋，進(jìn)行詳細(xì)敘述，分析其裂紋產(chǎn)生原因極其修理、預(yù)防措施等。

一．氣缸蓋裂紋產(chǎn)生的部位　

船用柴油機(jī)氣缸蓋的裂紋主要發(fā)生在氣缸蓋底面有應(yīng)力集中的地方，如孔與孔之間或孔的圓角處、冷卻水腔的截面變化處等。產(chǎn)生裂紋的具體部位則隨機(jī)型、氣缸蓋的結(jié)構(gòu)和材料不同而異。

　　船用四沖程柴油機(jī)在進(jìn)排氣閥孔和噴油器等孔之間以及閥座面上容易產(chǎn)生裂紋。當(dāng)裂紋產(chǎn)生在底面時，多為徑向裂紋；當(dāng)裂紋產(chǎn)生在外部時，裂紋大多是從中央噴油器孔向進(jìn)排氣閥孔擴(kuò)展，這是由于小孔處的應(yīng)力集中所致(如圖1所示)。

　　蘇爾壽RD和RND型柴油機(jī)氣缸蓋，裂紋大多發(fā)生中央小缸蓋底面上的噴油器孔、啟動閥孔、安全閥孔四周的圓角處，多沿徑向擴(kuò)展(如圖2所示)^[1]。

　　 MAN型柴油機(jī)氣缸蓋由上下兩部分組成，下半部分用耐熱合金鑄鐵制成，主要承受熱負(fù)荷；上半部分采用灰口鑄鐵制成，主要承受機(jī)械負(fù)荷，由于下半部分缸蓋冷卻水腔布置了環(huán)形冷卻水道，故裂紋多

2 氣缸蓋產(chǎn)生裂紋的原因

　　氣缸蓋產(chǎn)生裂紋是氣缸蓋較為常見的故障。氣缸蓋產(chǎn)生裂紋的根本原因是熱應(yīng)力和機(jī)械應(yīng)力周期性的作用。在交變的熱應(yīng)力和機(jī)械應(yīng)力的作用下，將產(chǎn)生疲勞裂紋，從而導(dǎo)致氣缸蓋裂紋。具體分析氣缸蓋產(chǎn)生裂紋主要有以下幾個方面原因。

1 結(jié)構(gòu)設(shè)計上的原因

　　氣缸蓋底面氣閥孔周圍之所以常產(chǎn)生裂紋，主要因為該處有較大的表面積，因此，受熱膨脹和冷卻時收縮速度都較大。例如，柴油機(jī)工作一段時間停車后，氣缸蓋溫度分布變化劇烈，熱量通過冷卻水和進(jìn)排氣通道迅速散發(fā)，所以在氣閥孔處容易產(chǎn)生裂縫。

　　再者，由于結(jié)構(gòu)或受力不合理、過度圓角太小等均會引起過大的機(jī)械應(yīng)力，從而導(dǎo)致裂紋。

2 材料和工藝上的原因

　　氣缸蓋材料選擇不當(dāng)，質(zhì)量不符合要求，鑄造時沒有很好地消除鑄造應(yīng)力，從而導(dǎo)致零件內(nèi)部有缺陷，從而使氣缸蓋在工作時容易產(chǎn)生裂紋。

3 裝配質(zhì)量上的原因

　　氣缸蓋螺栓不按規(guī)定交叉擰緊，或在發(fā)生氣缸蓋平面漏氣時擰緊該處的螺母來解決，都會造成氣缸蓋受力不均勻而產(chǎn)生裂紋。噴油器安裝不正確，會引起氣缸蓋底面局部變形，增大噴油器孔處所受的拉應(yīng)力，使之容易產(chǎn)生裂紋。

4 操作和管理上的原因

　　柴油機(jī)長期在超負(fù)荷條件下運(yùn)轉(zhuǎn)或在未充分暖缸的情況下突然增大負(fù)荷，使氣缸蓋從冷態(tài)急劇變?yōu)闊釕B(tài)，或在高溫下急劇冷卻而產(chǎn)生裂紋。

　　柴油機(jī)冷車啟動或啟動后加速太快，致使氣缸蓋底面與冷卻面溫差過大，熱應(yīng)力增大。故應(yīng)暖機(jī)后再啟動，啟動后待油、水溫度升高后方可加速；頻繁啟動、停車和長期超負(fù)荷運(yùn)轉(zhuǎn)均會使機(jī)械應(yīng)力和熱應(yīng)力增加；冷卻與潤滑不足或中斷，亦會導(dǎo)致機(jī)件過熱。

5 維護(hù)保養(yǎng)不良的原因

安裝氣缸蓋時未按規(guī)定上緊螺栓，個螺栓受力不均，使氣缸蓋產(chǎn)生過大的附加應(yīng)力；長期使用，未清潔冷卻水道，致使水腔積垢，影響散熱，引起過大的熱應(yīng)力，從而導(dǎo)致裂紋的產(chǎn)生。

3 氣缸蓋疲勞裂紋

1 熱疲勞裂紋

在底板受熱面“鼻梁區(qū)”產(chǎn)生熱疲勞裂紋是四沖程柴油機(jī)氣缸蓋較為常見的損壞形式。產(chǎn)生熱疲勞是由于受熱面的溫度超過材料的使用極限而發(fā)生蠕變，引起塑性變形。當(dāng)氣缸蓋在冷熱交替情況下工作時，受熱面的收縮性變形而受阻，從而產(chǎn)生殘余拉伸應(yīng)力。受熱面溫度超過極限溫度越多，運(yùn)行時間越長，殘余拉伸應(yīng)力就越大，最終因應(yīng)力的反復(fù)交變而出現(xiàn)疲勞裂紋。嚴(yán)重時可能經(jīng)過一次冷熱循環(huán)便出現(xiàn)裂紋^[5]。

2 機(jī)械疲勞裂紋

氣缸蓋底板在缸內(nèi)氣體壓力作用下發(fā)生周期性彎曲變形，其最大拉伸應(yīng)力發(fā)生于底板的內(nèi)側(cè)表面，閥孔尖角處應(yīng)力集中嚴(yán)重，再疊加上熱應(yīng)力（底板內(nèi)側(cè)表面所受熱應(yīng)力也為拉伸應(yīng)力）的作用，于是機(jī)械疲勞裂紋以此尖角為起點，沿閥孔周圍逐漸向受熱面擴(kuò)展而裂穿。

3 腐蝕疲勞裂紋

冷卻水中含有各種酸根離子和溶解氧將對金屬發(fā)生電化學(xué)腐蝕和氧化，金屬晶界是首先被腐蝕和氧化的部位。這將導(dǎo)致被腐蝕的表面變得粗糙并產(chǎn)生應(yīng)力集中，使材料的疲勞強(qiáng)度顯著下降。又由于氣缸蓋與氣缸套的圓錐部分配合不良，以及氣缸蓋螺栓的過分緊固，在柴油機(jī)運(yùn)轉(zhuǎn)時，使密封凸緣冷卻水側(cè)產(chǎn)生較大的脈動機(jī)械應(yīng)力。在腐蝕和較大的脈動機(jī)械應(yīng)力共同作用下，從冷卻水側(cè)開始發(fā)生腐蝕疲勞裂紋。

4 氣缸蓋裂紋檢查

　　通常，氣缸蓋裂紋都有一個產(chǎn)生和發(fā)展的過程，因此，平時認(rèn)真檢查、仔細(xì)觀察、及早發(fā)現(xiàn)，就能避免機(jī)損事故的發(fā)生。氣缸蓋裂紋檢查主要有３種方法：直接觀察法、著色探傷法、液壓試驗法。

1 直接觀察法

可根據(jù)冷卻水壓力表指針的擺動和膨脹水箱中水位的上下波動及有無氣泄、油漬等來判斷氣缸蓋是否有裂紋，還可通過示功閥檢查排氣是否冒白煙或有燃燒不良現(xiàn)象來判斷裂紋發(fā)生在哪一缸。經(jīng)常檢查曲軸箱中滑油油位的變化，若油位升高，油中水分明顯增加，則應(yīng)懷疑內(nèi)有裂紋。

2 著色探傷法

著色探傷滲透液是含有紅色顏料、溶劑和滲透劑等成分。具有滲透力強(qiáng)，滲透速度快，顯象時清晰醒目，洗滌性好，化學(xué)穩(wěn)定性好和無腐蝕或低毒等特點。顯象劑常為氧化鋅、氧化鎂或二氧化鈦等白色粉末和有機(jī)溶劑組成。顯象劑具有懸浮力好，與滲透液有明顯的襯度對比，顯示缺陷清晰，易于辨別，無腐蝕性等特點。

在著色探傷操作中有浸液法、刷涂法和噴涂法。內(nèi)壓式噴灌操作簡單，攜帶方便，廣為應(yīng)用。一組內(nèi)壓式噴灌各裝有清洗劑、滲透劑和顯象劑。檢驗時，先用清洗劑清潔零件待檢表面，然后噴涂一層滲透劑，依據(jù)材料不同有不同的滲透時間。如常溫下鋁、鎂合金鑄件約為15分鐘，鍛件和鋼鑄件應(yīng)不小于30分鐘；鋼焊件和焊縫有的可打60分鐘；塑料、玻璃、陶瓷等非金屬材料在5~30分鐘之間。滲透劑滲透時間對檢驗靈敏度影響很大。時間短，小缺陷難以發(fā)現(xiàn)，大缺陷顯示不全；時間長，難以清洗和檢驗效率低。在清洗掉表面滲透劑后再噴涂顯象劑，最后就可在白色襯底上顯示出紅色缺陷痕跡。

3 液壓試驗法

對于內(nèi)部微小裂紋，如冷卻水腔的微小裂紋，用著色法很難檢查，通常做液壓試驗才能檢查出來。

液壓試驗法實質(zhì)上是在模擬使用條件下對承壓零件材料內(nèi)部有無缺陷進(jìn)行檢驗的一種無損檢驗方法。它不需任何先進(jìn)的儀器，只用一般的專用夾具和具有壓力的氣體或液體。

液壓試驗前，將待檢零件上的孔、洞等全部堵塞，用專用夾具密封零件形成包括檢驗部位的密封空腔，注滿液體或氣體后完全封閉，按要求加壓至規(guī)定試驗壓力，保持一定時間后觀察零件外表面上有無液體滲出或氣體逸出，從而判斷零件有無裂紋。

試驗用液體可用水或油，也可用空氣，以有關(guān)要求而定。試驗壓力以零件工作條件而定。柴油機(jī)氣缸蓋冷卻水腔試驗壓力為0.7MPa，保持5分鐘。另外,航行中根據(jù)下列現(xiàn)象判斷燃燒室零件有無穿透性裂紋：

1)柴油機(jī)運(yùn)轉(zhuǎn)中，輪機(jī)員可根據(jù)氣缸或活塞冷卻水表壓力指針波動或膨脹水柜水位上下波動判斷零件有穿透性裂紋。因為當(dāng)氣缸蓋或氣缸套有穿透性裂紋時，燃燒室中的高壓燃?xì)饩蜁亓芽p進(jìn)入冷卻水腔，使冷卻水系統(tǒng)的壓力升高，壓力表指針和膨脹水柜水位均升高；當(dāng)氣缸排氣后壓力低于冷卻水壓力，冷卻水自裂縫進(jìn)入氣缸大量漏泄，冷卻水壓力急劇降低，壓力表指針和膨脹水柜水位又迅速降低。此外，還可從冷卻水溫升高，淡水消耗量增加，掃氣箱有水流出，膨脹水柜的透氣管有氣泡冒出和冷卻水中有油星等現(xiàn)象進(jìn)一步判斷。至于燃燒室中哪個零件裂穿則需要進(jìn)一步檢查。

2)啟動前進(jìn)行轉(zhuǎn)車和沖車時，輪機(jī)員應(yīng)打開示功閥觀察有無水氣或水珠噴出。如有水氣或水珠噴出表明燃燒室零件有穿透性裂紋或噴油器冷卻水漏泄，應(yīng)進(jìn)一步檢查和處理，否則缸內(nèi)積水較多直接啟動就會造成水擊事故。

3)曲柄箱（或循環(huán)油柜）中滑油量不正常增多或滑油中水分明顯增加，或滑油迅速乳化變質(zhì)均表明由于燃燒室部件有穿透性裂紋使冷卻水大量漏入。

4)吊缸檢修時，輪機(jī)員應(yīng)認(rèn)真仔細(xì)觀察各個零件，如發(fā)現(xiàn)活塞、氣缸套和氣缸蓋表面有銹痕，或活塞頂部積水等，說明燃燒室零件有穿透性裂紋^[8]。

4 其他方法

在檢修中，還可用熒光法、磁力探傷法、超聲波探傷法、射線探傷法等多種方法來檢查裂紋。此處不再一一敘述。

5 氣缸蓋裂紋的應(yīng)急修理

　　如果氣缸蓋裂紋程度較為嚴(yán)重，比如當(dāng)氣缸蓋的裂紋是裂穿性的，或者裂紋產(chǎn)生在關(guān)鍵部位，或者裂紋程度較為嚴(yán)重，這些情況無疑都必須更換氣缸蓋；但當(dāng)裂紋不嚴(yán)重或為了應(yīng)急或延長使用，可根據(jù)不同的場合選擇采用合適的方法進(jìn)行修理。

1 無機(jī)粘結(jié)劑修補(bǔ)法

　　這是一種最方便的方法，由于無機(jī)粘結(jié)劑能夠長期在500度高溫下工作，故可用于修補(bǔ)氣缸蓋底面裂紋。但是由于次溫度限制，所以建議在溫度處的裂紋采用有機(jī)粘結(jié)劑修補(bǔ)。

2 金屬扣合法

　　此法的特點是能保證強(qiáng)度又有很好的密封性。對于缸蓋底面裂紋的修理較其他方法可靠。為了增加密封性還可在裂紋處先涂一些無機(jī)粘合劑（如磷酸—氧化銅粘合劑）或者有機(jī)粘合劑，這樣效果更好^[3]。

3 鑲套修理法

主要用于氣缸蓋進(jìn)、排氣閥孔或噴油器孔內(nèi)的列修理。通常采用此修理后氣缸套可以使用兩年以上。襯套的材料一般采用不銹鋼或青銅，襯套端部與閥孔底部加紫銅墊以密封。

4 焊補(bǔ)法

當(dāng)裂紋較短時，在鏟去裂紋后進(jìn)行焊補(bǔ)。

焊補(bǔ)之前氣缸蓋應(yīng)預(yù)熱，焊前現(xiàn)在裂紋兩端各鉆一止裂孔，并沿裂紋開“U”型坡口，坡口底端應(yīng)成圓弧形。對于鑄鐵氣缸蓋，焊條可用鎳基鑄鐵焊條。每次焊接長度應(yīng)不超過30~40mm，并趁紅熱狀態(tài)用小錘輕輕敲擊整個焊道，以防止熱裂的產(chǎn)生。錘擊后立即用氧—乙炔火焰對準(zhǔn)焊縫加熱回火，回火溫度以焊縫表面呈暗紅色即可。

此方法對于焊補(bǔ)工藝要求嚴(yán)格，尤其是鑄鐵缸蓋焊接部位更應(yīng)特別小心，如焊前的預(yù)熱、焊后的退火處理及焊條的選擇等。因為稍有不當(dāng)就會在短期使用后因熱應(yīng)力而再次產(chǎn)生裂紋。

5 覆板修理法

此修理法僅適用于氣缸蓋外表面的修理，可以收到較好的效果。具體現(xiàn)在裂紋兩端鉆止裂孔，然后將鋼板覆蓋在裂紋部位上，再用螺釘固緊在氣缸蓋上。

氣缸蓋裂紋修理后，應(yīng)對冷卻水腔進(jìn)行0.7MPa的水壓試驗，以檢驗修理質(zhì)量。裂紋微小時采用銼刀、油石或風(fēng)沙輪等工具打磨裂紋處予以消除，經(jīng)無損探傷或水壓試驗檢驗合格后繼續(xù)使用。否則，繼續(xù)打磨、檢驗。若裂紋深達(dá)壁厚的3%以上時，停止打磨改用其他方法修理或報廢換新^[10]。

6 預(yù)防氣缸蓋產(chǎn)生裂紋的措施

為了保證柴油機(jī)的正常運(yùn)轉(zhuǎn)，作為柴油機(jī)的重要部件，預(yù)防其產(chǎn)生裂紋

更為重要，本文總結(jié)了以下幾條措施。

1 改善氣缸蓋材質(zhì)

通過改善材質(zhì)來減少氣缸蓋裂紋的產(chǎn)生是一個頗為有效的措施，如缸徑為400mm以下的氣缸蓋通常以灰鑄鐵普遍，而缸徑大于400mm時，由于灰鑄鐵經(jīng)不起大變形的長期考驗，因而以改用球墨鑄鐵為宜。

2 氣缸蓋設(shè)計優(yōu)化

如今年來許多中速柴油機(jī)都采用了雙層底氣缸蓋的結(jié)構(gòu)形式，它的設(shè)計觀點是把熱負(fù)荷和機(jī)械負(fù)荷分開來處理，采用較薄的底板可以提高冷卻效果，降低底板溫差，即用較薄的底板承受熱負(fù)荷，采用較厚的頂面板、中間隔板和圓周壁來提高剛度承受機(jī)械負(fù)荷，這樣使得機(jī)械應(yīng)力和熱應(yīng)力各得其所。

再如，為減少燃燒室壁上的熱應(yīng)力，缸蓋四周不能箍得太緊，缸蓋與缸套的定位凸肩要留有充分的余地，有如缸蓋閥門的過橋處常易產(chǎn)生裂紋，可在產(chǎn)生裂紋的部位預(yù)先銑去一部分金屬，以減輕應(yīng)力集中程度，從而減少了由于熱應(yīng)力過

大而導(dǎo)致裂紋產(chǎn)生的幾率。

3 加強(qiáng)對氣缸蓋過熱部位的冷卻

在柴油機(jī)投入運(yùn)行前，徹底清洗冷卻水腔，保證水路暢通?？刂评鋮s水的質(zhì)量，按說明書的規(guī)定控制好冷卻水的溫度、溫差和流量。在柴油機(jī)缸蓋冷卻水道內(nèi)增設(shè)擋水板，加快水流速度，且對水流給以必要的引導(dǎo)，使其定向地沖刷排氣門和噴油器使這些受熱嚴(yán)重的部件得較好的冷卻。避免長時間超負(fù)荷運(yùn)轉(zhuǎn)，防止過熱。當(dāng)采用外源驅(qū)動冷卻水泵時，柴油機(jī)停車后不應(yīng)過早停止冷卻水循環(huán)，以避免產(chǎn)生冷卻水“過后沸騰”現(xiàn)象。

4 輪機(jī)人員的操作管理科學(xué)化。

若對柴油機(jī)操作管理不當(dāng)，同樣會導(dǎo)致氣缸蓋產(chǎn)生裂紋。例如，柴油機(jī)冷車啟動或啟動后加速太快都容易產(chǎn)生裂紋。更甚者，輪機(jī)人員在沒有充分暖缸的情況下增加負(fù)荷，急劇調(diào)節(jié)冷卻水量，造成氣缸蓋“激碎”現(xiàn)象。這是由于受熱部件和冷卻液的溫度升高都需要有一個過程，如果加速過快，燃燒室部件就會因內(nèi)外壁溫差過大而產(chǎn)生過大的熱應(yīng)力，長此下去，就會產(chǎn)生裂紋，因此，輪機(jī)人員啟動主機(jī)前必須充分暖機(jī)，啟動后不可加速太快，待油溫和水溫升高后，方可加至全速，同時還要及時清除冷卻水腔中的水垢和鐵銹^[

^{Marine diesel engine cylinder head cracks and repairs

Gas cylinder head as diesel engine of fixed parts, is diesel engine combustion chamber of component. ship with diesel engine cylinder cover of structure type type range, random type different and different. in cylinder cover Shang usually layout row valve, and injector, and as shown in the Gong valve, and started valve, and safety valve and into exhaust of driven body,, cylinder cover of within cavity to layout corresponding of stream road, and duct, and and the cooling water cavity,. Therefore, common features of various types of cylinder head is a complex structure, more duct, wall thickness is uneven.
　　Cylinder head of diesel engine is not only complex, and the conditions of work, force is also very complex and harsh. Cylinder cover of end of surface directly and high temperature high pressure gas contact, not only was high temperature high pressure gas of corrosion and scour, and cycle to bear higher of machinery load and hot load; end of surface in gas of outbreak pressure role Xia produced is large of machinery stress, wall thick and temperature of does not uniform led to higher of hot stress; bolt of pre-tight force make cylinder head bear with pressure stress, and and gas pressure common role make cylinder head was bent role, in section changes Department will produced stress concentrated,. In addition, the corrosion of the cooling water Chamber was subjected to water.
　　Precisely because of such adverse working conditions of cylinder, cylinder head are easily damaged. Usually corrupted form: its underside and cooling cavity easy to crack, underside and the valve guide bushes also easy to wear, cooling water is corrosive.
This article mainly for marine diesel engine cylinder head most frequently appears to be corrupted--crack, detail, analysing their causes for crack repairs, as well as precautionary measures.
One. 　Crack of cylinder parts
Marine diesel cylinder head cracks occur mainly at the end of cylinder surface areas of stress concentration, such as a hole or holes with the holes of the round corners, sections such as change of cooling water jacket. Stochastic cracks of concrete parts, cylinder head structure and vary according to the material.
　　Four-stroke marine diesel engine and fuel injector hole in exhaust valve hole and prone to cracking on the seat surface. When a crack in the bottom surface, mostly radial crack; when the crack is external, cracks are mostly from the Central injector hole expansion to the vent holes, this was due to stress concentration due to small holes (as shown in Figure 1).
　　Sool tho RD and RND-type diesel engine cylinder cover, cracks occur mostly small cylinder heads on the underside of the central hole, activated valve, safety valve of injector hole holes around the corner, along the radial extension (as shown in Figure 2) [1].

　　 MAN diesel engine cylinder head is composed of two parts, the lower part is made of heat resistant alloys of cast iron, mainly under thermal load; upper part made of grey cast iron, primarily to bear external mechanical load, due to the lower part of the cylinder head cooling cavity layout of the circular cooling water line, so crack
2 reasons for cracks in cylinder
　　Cylinder head cracking was more common faults of cylinder. Cylinder head cracking due to thermal stress and mechanical stress a recurring role. Alternating thermal stress and under the effect of mechanical stress, fatigue cracks, resulting in cracks in cylinder. Specific analysis of cylinder head cracks the following main reasons.
1 structural design reasons
　　Around the hole of the bottom surface of cylinder valve is often cracking, mainly because it has a larger surface area, therefore, larger heat expansion and contraction when the cooling rate. For example, the diesel engine work for some time after stopping, cylinder head temperature distribution changes are severe, heat through the cooling and intake and exhaust channels quickly disseminated so prone to cracking at the valve hole.
　　Furthermore, as force unreasonable, excessive rounded structure or too small will cause excessive mechanical stress, resulting in cracks.
2 reasons for materials and workmanship
　　Improper cylinder head material selection, quality does not meet the requirements, casting without elimination of casting stress well, resulting in internal defective parts, so that when the cylinder head work is prone to cracking.
3 assembling quality reasons
　　Cylinder head bolts do not cross tighten as required, or in the event of cylinder flat leak tighten the nut to resolve, caused arising from the uneven force crack of cylinder. Fuel injector is not installed properly, will cause cylinder end of local deformation, increases the injector hole premises subject to tensile stress, making it prone to cracking.
4 operational and administrative reasons
　　Diesel engine under overload conditions, long running or in cases not fully heated cylinder suddenly increasing load, to make cylinder head sharply from cold to hot, or a sharp cooling in high temperature crack.
　　Diesel engine cold starts or accelerating too fast after starting, underside and excessive temperature difference of cooling surface of cylinder, heat stress increases. It should be after warm start, starts oil, water temperatures must be accelerated; frequent starting, stopping, and overstretched will enable increased mechanical stress and thermal stress; inadequate cooling and lubrication or break, could also result in part from overheating.
5 causes of poor maintenance
Do not tighten bolts as required when installing the cylinder head, bolt force does not, make additional stress in cylinder heads are too large; long-term use, is not clean cooling water road, water cavity scale, affecting the thermal, thermal stress caused by too large, resulting in cracks.
Fatigue crack propagation in 3 cylinder
1 thermal fatigue crack
The baseboard heating surface "nose area" thermal fatigue crack generated is cylinder four-stroke diesel engines of the more common forms of damage. Thermal fatigue due to the heated surface temperature creep occurs beyond the limits and use of materials, causing plastic deformation. When the cylinder head in alternating hot and cold case work, was hampered by shrinkage deformation of heated surface, resulting in residual tensile stresses. Heated surface temperature exceeds the limit, the more long running time, residual tensile stresses, the larger, eventually suffer fatigue from repeated alternating stress cracks. Serious cracks after a hot and cold cycles occur when [5].
2 mechanical fatigue crack
Gas cylinder head backplane in cylinder within gas pressure role Xia occurs cyclical bent deformation, its maximum stretch stress occurs Yu backplane of inside surface, valve hole Tsim corner Department stress concentrated serious, again overlay Shang hot stress (backplane inside surface by heated stress also for stretch stress) of role, so machinery fatigue crack to this Tsim corner for starting point, along valve hole around gradually to heated surface extended and crack wearing.
3 corrosion fatigue crack propagation
Cooling water contains dissolved oxygen ion and will be on electro-chemical corrosion and oxidation of metals, corrosion and oxidation of metal grain boundaries is first to be part of. This will lead to corrosion of the surface became rough and produces stress concentration, fatigue strength of the material declined significantly. Due to the tapered part of the cylinder head and cylinder misfit, as well as the excessive tightening of cylinder head bolts, diesel engine operation, make the seal flange cooling water larger pulse of mechanical stress. Corrosion and large fluctuation under mechanical stress, corrosion fatigue crack propagation occurs starting from the cooling water.
4 cylinder head crack check
　　Typically, cylinder head cracks has a generation and development process and, therefore, normally careful checks, careful observation, early detection, will be able to avoid engine damage occurs. Cylinder head crack check there are 3 main methods: direct observation method, dye penetration test method, the hydraulic pressure test method.
1 direct observation method
According to cooling water pressure gauge pointer swings and fluctuations of the water level in the expansion tank and there is no gas, oil stains and other relief to determine whether there are cracks in cylinder, you can also check for white smoke exhaust whether to risk by indicator valve or burning undesirable phenomena to determine crack cylinder in which it occurs. Always check the oil in the crankcase oil change if oil rises, marked increase in moisture in oil, you should suspect has cracked.
2 penetrant inspection method
Penetrant inspection penetrants contain red pigment, solvent and penetrating agents and other ingredients. With strong penetration, penetration speed, when the picture clear and eye-catching, good wash, chemical stability and corrosion-free or low-toxicit, y and so on. Photographic developer is often white powder, such as zinc oxide, magnesium oxide, or titanium dioxide and organic solvent composition. Photographic developer has a better suspension, compared with clear contrast penetrant, display defects in clear, easy to identify, non-corrosive and so on.
IB in a dye penetration test operation, brush and spray law. Pressure sprinkler operation in a simple, easy to carry, and wide application. Sprinkler is equipped with a set of internal pressure cleaner, penetrant and Imaging agents. Inspection, using the surface of the cleaning agents to clean the part to be ch, ecked first, then spray a layer of penetrating agent, based on different materials have different penetration time. Such as aluminum and magnesium alloy castings at room temperature about 15 minutes, steel castings, forgings and shall be not less than 30 minutes; steel and welds have to play 60 minutes; non-metallic materials such as plastic, glass, ceramic 5-30 minutes. Penetrant osmotic effects of time on testing sensitivity is very big. Time is short, hard to find small defects, defect does not display; a long, difficult to clean and test efficiency is low. Penetrant on the surface and then spray the cleaning Imaging agent, and finally to display red on a white substrate defects traces.
3 hydraulic test method
For an internal micro-cracks, such as cooling water jacket of tiny cracks, colouring it is difficult to check, usually do the hydraulic pressure test to check it out.
Hydraulic test method is essentially a pressurized parts in a simulated use conditions there are no material defect inspection of a non-destructive test methods. It does not require any advanced equipment, only with the special clamp and pressure of gases or liquids.
Before the hydraulic pressure test, to be checked on a part of the hole, hole, and all blocked, sealed with special fixture parts sealed cavity formation including test site, fully closed after filled with liquid or gas, pressurized as required to provide testing pressure, maintained a certain amount of time, the observations on the outer surface of the part have no drainage or escape of gases to determine parts without cracks.
Test of liquid water or oil, air can also be used, in respect of the requirements may be. Test pressure to parts of working conditions may be. Pressures of test cylinder head of diesel engine cooling water for 0.7MPa for 5 minutes. In addition, navigation based on the following combustion chamber part no penetrating cracks:
1) diesel engines in operation, engineer according to the cylinder or piston cooling water table fluctuations in pressure pointers or expansion tank water level fluctuations determine parts of penetrating cracks. Because Dang gas cylinder head or cylinder sets has penetrating sexual crack Shi, combustion chamber in the of high pressure gas on will along crack entered cooling water cavity, makes cooling water system of pressure rise, pressure table pointer and expansion tank water level are rise; Dang cylinder exhaust Hou pressure below cooling water pressure, cooling water since crack entered cylinder large leak, cooling water pressure sharp reduced, pressure table pointer and expansion tank water level also quickly reduced. In addition, also available from the cooling water temperature rises, increased consumption of fresh water, water outflow scavenging box, vent tube air bubbles out of the expansion tank and cooling water in the soil further judgement. As to which parts through crack in the combustion chambers you will need further checking.
2) interchange and red car prior to commencement, engineer observation indicator valve should be opened for any moisture or water spray. If moisture or water spray out showed that combustion chamber cooling water leak parts of penetrating cracks or fuel injector, should be further examined and processed, otherwise stagnant water more directly in the cylinder start would result in accidents of water hammer.
3) crank case (or cycle oil tank), oil does not increase in the normal or apparent increase in moisture in oil, or oil emulsion deterioration suggest that rapidly cooling water due to combustion chamber components of penetrating cracks a lot of leaking into the.
4) when a top overhaul, engineer should be carefully observed the various parts, such as piston, cylinder and the cylinder head surface rust found trace amounts of, or top of the piston stagnant water, instructions parts of penetrating cracks of combustion chamber [8].
4 other methods
In inspection and repair, also use fluorescent method, magnetic flaw detection, ultrasonic, radiographic law in several ways, such as to check for cracks. Here are no longer described one by one.
5 emergency repair of cylinder head cracks
　　If the cylinder head cracks in more serious, such as when a cylinder head crack crack is wearing, or cracks in the key parts, more serious or crack, these circumstances undoubtedly cylinder must be replaced; but when the crack is not serious or emergency or extended use, using appropriate methods can be selected according to different occasions for repairs.
1 inorganic Binder for repairing method
　　This is one of the most convenient method, because inorganic Binder for long-term work in a 500 degree high temperature, it can be used to repair bottom crack in cylinder. But due to the temperature limit, so organic binder is cracked at the temperature recommended patches.
2 metalock
　　Features of this law is to ensure strength and good sealing. For reliable repair the bottom face of the cylinder head cracking than other methods. In order to increase the seal can also crack at the first coated inorganic binders (such as phosphoric acid-copper oxide adhesive) or organic binders, so better [3].
3 mounting repair law
Mainly for cylinder Gaijin, vent holes or injector hole repair. This repair is usually cylinder sleeves can be used for more than two years after. Materials generally use stainless steel or bronze bushing, bushing tip and bottom plus copper gasket to seal the valve hole.
4 welding method
When you crack a relatively short, spade came to crack in welding.
Welding cylinder should warm up before, now before the welding crack crack drilled a hole at each end and along the cracked open "u" Groove, Groove bottom should be curved. Cast iron cylinder head and electrodes available nickel-base cast iron electrode. Each time the length should not exceed 30~40mm, and taking advantage of glowing red with a small hammer gently tap the entire welding road, to prevent hot cracking formation. Immediately after hammering with oxygen-acetylene flame at the weld heating tempering, tempering temperature weld surface is dark red.
This method for welding technology requires strict, particularly cast iron cylinder head welded joints with extreme caution, such as preheating before welding, annealing after welding and electrode selection, and so on. Because after the somewhat inappropriate in the short term to use crack again due to thermal stress.
5 repair method of review panel
This repair method applies only to the outer surface of the cylinder head repairs, you can receive better results. Concrete crack drill holes at both ends now, and then steel parts covered in cracks, then tighten the screw on the cylinder head.
Cylinder head cracks repaired, should 0.7MPa the cooling water jacket hydraulic pressure test to verify repair quality. Crack file used in the tiny, oil tools such as stone or sand wheel grinding cracks to be eliminated, NDT or continued use after hydraulic pressure test and inspection. Otherwise, continue polishing, testing. If the above 3% of crack depth of wall thickness, stop grinding to switch to other methods to repair or scrap-for-new [10].
6 measures to prevent cracks in cylinder
In order to ensure normal operation of diesel engines, diesel engine as an important part to prevent its cracking
More importantly, this article summarizes several of the following measures.
1 improved cylinder head material
By improving the material to reduce the generation of cracks in cylinder is a fairly effective measures, such as cylinder 400mm cylinder head below the usually gray cast iron is most common}