NGW行星齒輪減速器--軸的設計

1、目 錄第一章 緒論21.1 行星齒輪傳動的特點21.2 本文的主要內容3第二章 NGW行星齒輪減速器結構設計32.1 設計技術參數(shù)32.2 機構簡圖確定32.3 齒形與精度42.4 齒輪材料及其性能4第三章 齒輪的優(yōu)化設計43.1 齒輪的設計43.11配齒數(shù)43.12初步計算齒輪主要參數(shù)53.13幾何尺寸計算63.2 重合度計算73.2 齒輪嚙合效率計算73.4 疲勞強度校核83.41外嚙合83.42內嚙合13第四章 其他零件的設計144.1 軸承的設計144.2 行星架的設計15第五章 輸入軸的優(yōu)化設計155.1 裝配方案的選擇155.2 尺寸設計165.21初步確定軸的最小直徑165.22

2、根據(jù)軸向定位要求確定軸的各段直徑和長度175.23軸上零件軸向定位175.24確定軸上圓角和倒角尺寸185.3 輸入軸的受力分析185.31求輸入軸上的功率P、轉速n和轉矩T185.32求作用在太陽輪上的力185.33求軸上的載荷195.4按彎扭合成應力校核軸的強度215.5精確校核軸的疲勞強度225.6 按靜強度條件進行校核28第六章 Solidworks出圖30參考文獻34第一章 緒論漸開線行星齒輪減速器是一種至少有一個齒輪繞著位置固定的幾何軸線作圓周運動的齒輪傳動，這種傳動通常用內嚙合且多采用幾個行星輪同時傳遞載荷，以使功率分流。漸開線行星齒輪傳動具有以下優(yōu)點:傳動比范圍大、結構緊湊、體

3、積和質量小、效率普遍較高、噪音低以及運轉平穩(wěn)等，因此被廣泛應用于起重、冶金、工程機械、運輸、航空、機床、電工機械以及國防工業(yè)等部門作為減速、變速或增速齒輪傳動裝置。漸開線行星齒輪減速器所用的行星齒輪傳動類型很多，按傳動機構中齒輪的嚙合方式分為:NGW、NW、NN、NGWN、ZU飛VGW、W.W等，其中的字母表示:N內嚙合，W外嚙合，G內外嚙合公用行星齒輪，ZU錐齒輪。1.1 行星齒輪傳動的特點行星齒輪傳動與其他形式的齒輪傳動相比有如下幾個特點:（1）體積小、重量輕、結構緊湊、傳遞功率大、承載能力高，這個特點是由行星齒輪傳動的結構等內在因素決定的。（2）傳動比大 只要適當?shù)倪x擇行星傳動的類型及配

4、齒方案，就可以利用很少的幾個齒輪而得到很大的傳動比。在不作為動力傳動而主要用以傳遞運動的行星機構中，其傳動比可達到幾千。此外，行星齒輪傳動由于它的三個基本構件都可以傳動，故可以實現(xiàn)運動的合成與分解，以及有級和無級變速傳動等復雜的運動。（3）傳動效率高 由于行星齒輪傳動采用了對稱的分流傳動結構，即它具有數(shù)個均勻分布的行星齒輪，使作用于中心輪和轉臂軸承中的反作用力相互平衡，有利于提高傳動效率。在傳動類型選擇恰當、結構布置合理的情況下，其效率可達0.970.99。（4）運動平穩(wěn)、抗沖擊和振動的能力較強由于采用數(shù)個相同的行星輪，均勻分布于中心輪周圍，從而可使行星輪與轉臂的慣性力相互平衡。同時，也使參與

5、嚙合的齒數(shù)增多，故行星齒輪傳動的運動平穩(wěn)，抗沖擊和振動的能力較強，工作較可靠。在具有上述特點和優(yōu)越性的同時，行星齒輪傳動也存在一些缺點，如結構形式比定軸齒輪傳動復雜；對制造質量要求較高；由于體積較小、散熱面積小導致油溫升高，故要求嚴格的潤滑與冷卻裝置等。行星齒輪傳動的設計進行研究，對促進技術進步和國民經(jīng)濟的發(fā)展具有重要的理論和實用意義。1.2 本文的主要內容NGW型行星齒輪傳動機構的傳動原理:當高速軸由電動機驅動時，帶動太陽輪回轉，再帶動行星輪轉動，由于內齒圈固定不動，便驅動行星架作輸出運動，行星輪在行星架上既作自轉又作公轉，以此同樣的結構組成二級、三級或多級傳動。NGW型行星齒輪傳動機構主要

6、由太陽輪、行星輪、內齒圈及行星架所組成，以基本構件命名，又稱為ZK-H型行星齒輪傳動機構。本設計的主要內容是單級NGW型行星減速器的設計。第二章 NGW行星齒輪減速器結構設計2.1 設計技術參數(shù) 已知輸入功率30KW,輸入轉速100r/min，傳動比6，每天工作16小時，使用壽命10年2.2 機構簡圖確定 減速器傳動比i=6,故屬于1級NGW型行星傳動系統(tǒng)(如圖2-1)。圖 2-1查書漸開線行星齒輪傳動設計書表4-1確定=2或3，從提高傳動裝置承載力，減小尺寸和重量出發(fā)，取=3。 計算系統(tǒng)自由度 W=3*3-2*3-2=12.3 齒形與精度因屬于低速傳動，以及方便加工，故采用齒形角為20

7、86;，直齒傳動，精度定位6級。2.4 齒輪材料及其性能太陽輪和行星輪采用硬齒面，內齒輪采用軟齒面，以提高承載能力，減小尺寸。材料選擇見表2-1。表2-1 齒輪材料及其性能齒輪材料熱處理 (N/mm²) (N/mm²)加工精度太陽輪20CrMnTi滲碳淬火HRC58 6214003506級行星輪245內齒輪40Cr調制HB2622936502207級第三章 齒輪的優(yōu)化設計3.1 齒輪的設計3.11配齒數(shù)采用比例法：按齒面硬度HRC=60，查 漸開線行星齒輪傳動設計 書圖4-7a的，又，取。 由傳動比條件知 計算內齒輪和行星齒輪齒數(shù)：3.12初步計算齒輪主要參數(shù)（1）按齒面接

8、觸強度計算太陽輪分度圓直徑用式進行計算，相關系數(shù)取值如表3-1。其中：太陽輪傳遞的扭矩：則太陽輪分度圓直徑為：表3-1 齒面接觸強度有關系數(shù)代號名 稱說 明取 值算式系數(shù)直齒輪768使用系數(shù)表6-5，中等沖擊1.25行星輪間載荷分配系數(shù)表7-2，太陽輪浮動，6級精度1.05綜合系數(shù)表6-4，高精度，硬齒面1.8小齒輪齒寬系數(shù)表6-30.7實驗齒輪的接觸疲勞極限圖6-161400 以上均為在書漸開線行星齒輪傳動設計上查得(2)按彎曲強度初算模數(shù)用式進行計算。式中相關系數(shù)同表3-1，其余系數(shù)取值如表3-2。因為，所以應按行星輪計算模數(shù)：表3-2 彎曲強度有關系數(shù)代號名 稱說 明取 值算式系數(shù)直齒輪

9、12.1行星輪間載荷分配系數(shù)1.075綜合系數(shù)表6-4，高精度，1.6齒形系數(shù)圖6-25，按x=0查值3.18齒形系數(shù)圖6-25，按x=0查值2.45 以上均為在書漸開線行星齒輪傳動設計上查得若取莫屬，則太陽輪直徑：接觸強度初算結果接近，故初定按進行接觸和彎曲疲勞強度校核計算。3.13幾何尺寸計算將分度圓直徑、節(jié)圓直徑、齒頂圓直徑的計算值列于表3-3。表3-3 齒輪幾何尺寸齒輪分度圓直徑節(jié)圓直徑齒頂圓直徑太陽輪行星輪外嚙合內嚙合內齒輪3.2 重合度計算外嚙合： 內嚙合：3.2 齒輪嚙合效率計算按公式進行計算。式中為轉化機構的效率，可用Kyp計算法確定。查漸開線行星齒輪傳動設計中圖3-3a、b（

10、取µ=0.06，因齒輪精度高）得各嚙合副的效率為，轉化機構效率為：轉化機構傳動比：則 .3.4 疲勞強度校核3.41外嚙合（1）齒面接觸疲勞強度用式，計算接觸應力，用式計算其許用應力。三式中的參數(shù)和系數(shù)取值如表3-4。表3-4 外嚙合接觸強度有關參數(shù)和系數(shù)代號名 稱說 明取值使用系數(shù)按中等沖擊查表6-51.25動載荷系數(shù)，6級精度，查圖6-5b1.005齒向載荷分布系數(shù)查圖6-6得，取，由式（6-25）得1.114齒間載荷分配系數(shù)按，6級精度，硬齒面，查圖6-91行星輪間載荷不均衡系數(shù)太陽輪浮動，查表7-21.05節(jié)點區(qū)域系數(shù)查圖6-102.5彈性系數(shù)查表6-7189.8重合度系數(shù)，

11、查圖6-110.89螺旋角系數(shù)直齒，1分度圓上的切向力18723.53Nb工作齒寬72 mmu齒比數(shù)2壽命系數(shù)按工作10年每年365天，每天16小時計算應力循環(huán)次數(shù)1.03潤滑油系數(shù)HRC=HV713,v=0.445m/s,查表8-10用中型極壓油，1.05速度系數(shù)查圖6-200.88粗造度系數(shù)按，查圖6-211.03工作硬化系數(shù)兩齒輪均為硬齒面，圖6-221尺寸系數(shù)m61最小安全系數(shù)按可靠度查表6-81.25接觸疲勞極限查圖6-161400以上均為在書漸開線行星齒輪傳動設計上查得接觸應力基本值：接觸應力：許用接觸應力：因，故接觸強度通過。（2）齒根彎曲疲勞強度齒根彎曲疲勞應力及其許用應力，用

12、式和計算。并分別對太陽輪和行星輪進行校核。對于表3-4中未出現(xiàn)的參數(shù)和系數(shù)取值如表3-5。太陽輪：彎曲應力基本值：彎曲應力：許用彎曲應力:因，故太陽輪彎曲強度通過。行星輪：因，故行星輪彎曲強度通過。表3-5 外嚙合齒根彎曲強度的有關參數(shù)和系數(shù)代號名 稱說 明取值齒向載荷分布系數(shù)由，b/m=12，查圖6-23得，由式（6-38）得1.076齒間載荷分配系數(shù)1行星輪間載荷分配系數(shù)按式（7-43），1.075太陽輪齒形系數(shù)，查圖6-252.95行星輪齒形系數(shù)，查圖6-252.45太陽輪應力修正系數(shù)查圖6-271.55行星輪應力修正系數(shù)查圖6-271.68重合度系數(shù)式(6-40), 0.719彎曲壽命

13、系數(shù)1試驗齒輪應力修正系數(shù)按所給的區(qū)域圖取時2太陽輪齒根圓角敏感系數(shù)查圖6-350.95行星輪齒根圓角敏感系數(shù)查圖6-350.96齒根表面形狀系數(shù)，查圖6-361.045最小安全系數(shù)按高可靠度，查表6-81.6以上均為在書漸開線行星齒輪傳動設計上查得3.42內嚙合（1）齒面接觸疲勞強度同外嚙合齒面接觸疲勞強度所用公式相同，其中與外嚙合取值不同的參數(shù)為 。則：因，故接觸強度通過。（2）齒根彎曲疲勞強度只需計算內齒輪。計算公式與外嚙合齒根彎曲疲勞強度相同，其中取值與外嚙合不同的系數(shù)為。則：因,故彎曲強度通過。以上計算說明齒輪的承載能力足夠。第四章 其他零件的設計4.1 軸承的設計考慮到采用直齒輪傳

14、動，以及為了加工和裝配方便，擬用中空式行星輪，內孔中裝一個深溝球軸承，心軸固定在行星架上。用式計算軸承的動負荷，其中系數(shù)確定如表4-1。選用深溝球軸承61914，軸承的額定動負荷滿足條件。表4-1 軸承動負荷相關系數(shù)代號名 稱說 明取值負荷性質系數(shù)表9-18，中等沖擊1.25齒輪系數(shù)查表9-191.06安裝部位系數(shù)表9-20，對稱1.1工作情況系數(shù)1.4575溫度系數(shù)一般低速傳動1壽命系數(shù)更換期1.5年，2.36速度系數(shù)式（9-62）4.27行星架傳遞扭矩16964.75N·mP當量載荷式（9-63），19404.13N以上均為在書漸開線行星齒輪傳動設計上查得4.2 行星架的設計采用

15、雙壁整體式行星架，一端有浮動內齒圈。按經(jīng)驗取壁厚。兩壁之間的扇形斷面連接板其慣性中心所在半徑按式計算。行星架外徑b=251.84 mm，a=78.46mm， 按上述經(jīng)驗數(shù)據(jù)擬定的行星架尺寸，不必作強度計算。至此，NGW行星傳動系統(tǒng)設計完成第五章 輸入軸的優(yōu)化設計5.1 裝配方案的選擇輸入軸的裝配方案如圖6-1所示圖 6-15.2 尺寸設計5.21初步確定軸的最小直徑先按式初步估算軸的最小直徑。選取軸的材料為45鋼，調至處理。根據(jù)相關圖表，由于軸無軸向載荷，故A取較大值，即A=118，于是得：輸入軸的最小直徑顯然是安裝聯(lián)軸器處的軸的直徑（如圖6-1）。為了使所選的軸的直徑與聯(lián)軸器的孔徑相適應，故

16、需同時選取聯(lián)軸器型號。聯(lián)軸器計算轉矩，查相關圖標，考慮到轉矩變化很小，故取，則：按照計算轉矩應小于聯(lián)軸器公稱轉矩的條件，且查相關手冊，選用LH7型彈性柱銷聯(lián)軸器，其公稱轉矩為630000 N·mm。半聯(lián)軸器孔徑d=80 mm，故取，半聯(lián)軸器長度L=172 mm，半聯(lián)軸器與軸配合的轂孔長度。5.22根據(jù)軸向定位要求確定軸的各段直徑和長度（1）為了滿足半聯(lián)軸器軸向定位要求，-軸段右端需制出一軸肩，一般定位軸肩的高度為故取-段的直徑為。半聯(lián)軸器與軸配合的轂孔長度，為了保證軸向定位可靠和軸端擋圈只壓在半聯(lián)軸器上而不壓在軸的端面上，故-段的長度應比轂孔長度短23 mm，故取。（2）初步選擇滾動

17、軸承。因軸承只受徑向力作用，故選用深溝球軸承。參照工作要求并根據(jù)，由軸承產品目錄中初步選取0基本游隙組、標準精度級的深溝球軸承61919，其尺寸為d×D×B=95 mm×130 mm×18 mm。右端深溝球軸承采用軸肩進行軸向定位，因為滾動軸承的定位軸肩高度必須低于軸承內圈端面高度，查相關手冊知深溝球軸承61919內圈，故取。（3）為了軸承端蓋的方便拆裝及便于對軸承添加潤滑脂的要求，查得相關手冊取端蓋的外端面與半聯(lián)軸器右端面之間的距離l=36 mm；考慮到軸承端蓋和前機蓋的寬度，故取。（4）因該行星輪傳動系統(tǒng)為太陽輪浮動，故輸入軸的-段與太陽輪通過花鍵連

18、接，查相關手冊選取小徑d=92的花鍵，故-段直徑為；為了保證太陽輪和輸入軸通過花鍵的裝配，故??；為了保證輸入軸的正常裝配，取。（可參照附錄-行星輪傳動系統(tǒng)裝配圖）5.23軸上零件軸向定位半聯(lián)軸器與軸的軸向定位采用平鍵連接，太陽輪與軸的軸向定位采用花鍵連接。根據(jù)查相關手冊，選用平鍵b×h×=22 mm×14 mm×110 mm；選用花鍵為N×d×D×B=10 mm×92 mm×98 mm×14 mm。5.24確定軸上圓角和倒角尺寸查得相關手冊，輸入軸-段軸端倒角為2×45°，-

19、段軸端倒角為2.5×45°，截面處軸肩圓角為R2，其余軸肩圓角為R2.5。5.3 輸入軸的受力分析5.31求輸入軸上的功率P、轉速n和轉矩T已知P=30 KW,n=100 r/min則 5.32求作用在太陽輪上的力已知太陽輪分度圓直徑為：太陽輪上所受的徑向力如圖6-2（按受載不均勻條件下的合成計算不定向）圖6-2假設行星輪C1與太陽輪a嚙合傳遞轉矩為：（不均勻條件下最大轉矩）則行星輪C2、C3與太陽輪a嚙合傳遞的轉矩為：太陽輪與行星輪嚙合處圓周力如圖6-2所示，則有：其徑向力為：則太陽輪所受圓周力合力、徑向力合力如圖6-3所示。圖6-3徑向力：（方向不定）圓周力：（與垂直）

20、5.33求軸上的載荷首先根據(jù)軸的結構圖做出軸的受力簡圖如何6-4a；做出軸的彎矩圖和扭矩圖如圖6-4所示（1）作為簡支梁的軸的支撐跨距：（根據(jù)軸與軸上零件的裝配關系見附錄4）（2）左端聯(lián)軸器屬于有彈性元件的彈性柱銷聯(lián)軸器，有方向不定徑向力，?。ㄈ鐖D6-4a），則：（3）軸xoz平面上受力分布及彎矩圖（如圖6-4b）:則D點處的彎矩（4）軸xoy平面上受力分布及彎矩圖（如圖6-4c）：則D點的彎矩（5）初步合成彎矩圖（如圖6-4d）（6）與聯(lián)軸器徑向力在同一平面內的受力分布及彎矩圖（如圖6-4e）：則該平面內彎矩為（7）合成彎矩圖如圖（6-4f）所示（8）扭矩圖如圖（6-4g）所示：T=2865

21、000 N·mm圖 6-45.4按彎扭合成應力校核軸的強度根據(jù)式進行校核。其中，因為軸單向旋轉，扭轉切應力為脈動循環(huán)應力，取=0.6；為軸的計算應力；M為軸所受的彎矩；T為軸所受的扭矩；W為軸的抗彎截面系數(shù)，因為截面C為圓形，所以W=0.1d³。（1）C、D兩截面軸徑相同，又,故校核D截面即可：則軸的計算應力;前已選定軸的材料為45鋼，調至處理，查相關手冊查得。因為，故截面C處安全。（2）由于截面B左側不受扭矩作用，故只要校核截面B右側即可。則軸的計算應力為：故截面B右側安全5.5精確校核軸的疲勞強度（1）截面處校核 截面左側抗彎截面系數(shù)抗扭截面系數(shù)截面左側的彎矩M為截面上

22、的扭矩T為T=2865000 N·mm截面上的彎曲應力截面上的扭轉切應力軸的材料為45鋼，調制處理，查相關手冊查得:抗拉強度極限彎曲疲勞極限剪切疲勞極限截面上由于軸肩而形成的理論應力集中系數(shù)和可按相關手冊查取。因r/d=2.0/80=0.025，D/d=95/80=1.19,經(jīng)過插值后可查得：又由相關手冊可查得軸的材料的敏感系數(shù)為：故有效應力集中為：根據(jù)相關手冊查得尺寸系數(shù)，表面質量系數(shù)為軸按磨削加工，則表面質量系數(shù)為；軸未經(jīng)表面強化處理，即，則綜合系數(shù)為：又由碳鋼的特性系數(shù)：，取，取于是，計算安全系數(shù)的值，得：故可知其安全。 截面右側抗彎截面系數(shù)抗扭截面系數(shù)截面右側的彎矩M為截面上

23、的扭矩T為T=2865000 N·mm截面上的彎曲應力截面上的扭轉切應力因r/d=2.5/95=0.026，D/d=95/80=1.19,經(jīng)過插值后可查得：有效應力集中為根據(jù)相關手冊查得尺寸系數(shù)，表面質量系數(shù)為，則綜合系數(shù)為：于是，計算安全系數(shù)的值，得： 故可知其安全。（2）截面處校核 截面左側抗彎截面系數(shù)抗扭截面系數(shù)截面左側的彎矩M為：截面上的扭矩T為T=2865000 N·mm截面上的彎曲應力截面上的扭轉切應力因r/d=2.5/95=0.026，D/d=103/95=1.08,經(jīng)過插值后可查得：有效應力集中為:根據(jù)相關手冊查得尺寸系數(shù)，表面質量系數(shù)為，則綜合系數(shù)為：于是

24、，計算安全系數(shù)的值，得：故可知其安全。 截面右側抗彎截面系數(shù)抗扭截面系數(shù)截面右側的彎矩M為：截面上的扭矩T為T=2865000 N·mm截面上的彎曲應力截面上的扭轉切應力因r/d=2.5/103=0.024，D/d=103/95=1.08,經(jīng)過插值后可查得： 有效應力集中為根據(jù)相關手冊查得尺寸系數(shù)，表面質量系數(shù)為，則綜合系數(shù)為：于是，計算安全系數(shù)的值，得： 故可知其安全。(3) 截面處校核 截面左側抗彎截面系數(shù)抗扭截面系數(shù)截面左側的彎矩M為：截面上的扭矩T為T=2865000 N·mm截面上的彎曲應力截面上的扭轉切應力因r/d=2.5/103=0.024，D/d=103/9

25、5=1.08,經(jīng)過插值后可查得： 有效應力集中為根據(jù)相關手冊查得尺寸系數(shù)，表面質量系數(shù)為，則綜合系數(shù)為：于是，計算安全系數(shù)的值，得：故可知其安全。 截面右側抗彎截面系數(shù)抗扭截面系數(shù)截面左側的彎矩M為：截面上的扭矩T為T=2865000 N·mm截面上的彎曲應力截面上的扭轉切應力因r/d=2.5/92=0.027，D/d=103/95=1.08,經(jīng)過插值后可查得：有效應力集中為：根據(jù)相關手冊查得尺寸系數(shù)，表面質量系數(shù)為，則綜合系數(shù)為：于是，計算安全系數(shù)的值，得：故可知其安全。5.6 按靜強度條件進行校核(1)截面C處靜強度校核最大彎曲應力最大扭轉應力因軸的材料為45鋼調制處理，查相關手

26、冊查得:抗拉強度極限，抗彎屈服強度極限抗扭屈服極限，取：因，有，取，則按屈服強度設計的安全系數(shù)：故安全。（2）截面D處按靜強度條件進行校核最大彎曲應力最大扭轉應力按屈服強度設計的安全系數(shù)：故安全。至此，軸的設計完成。第六章 Solidworks出圖 太陽輪附錄2 行星輪附錄3 內齒輪輸入軸裝配體圖輸入軸工程圖裝配體工程圖參考文獻1 馬從謙，陳自修，張文照，張展，蔣學全，吳中心.漸開線行星齒輪傳動設計M.機械工業(yè)出版社,1987.2 孫恒,陳作模,葛文杰.機械原理M.7版.北京:高等教育出版社.20103 濮良貴,紀名剛,陳國定,吳立言.機械設計M.8版.北京高等教育出版社,2011.4 任繼生

27、,唐道武,馬克新.機械設計機械設計基礎課程設計M.中國礦業(yè)大學出版社,2009.外文資料翻譯PLAIN CARBON STEELAny steel-making process is capable of producing a product that has 0.05% or less carbon. With this small amount of carbon, the properties approach of pure iron with maximum ductility and minimum strength. Maximum ductility is desirable

28、 from the standpoint of ease in deformation processing and service use. Minimum strength is desirable for deformation processing. However, higher strengths than that obtainable with this low carbon are desirable from the standpoint of product design. The most practical means of increasing the streng

29、th is by the addition or retention of some carbon. However, it should be fully understood that any increase of strength over that pure iron can be obtained only at the expense of some loss of ductility, and the final choice is always a compromise of some degree. Because of the difficulty of composit

30、ion control or the additional operation of increasing carbon content, the cost of higher carbon, higher strength steel is greater than of low carbon.Plain Carbon Steels Most Used. Because of their low cost, the majority of steels used are plain carbon steels. These consist of iron combined with carb

31、on concentrated in there ranges classed as low carbon,medium carbon, and high carbon. With the exception of manganese used to control sulphur, other elements are present only in small enough quantities to be considered as impurities, though in some cases they may have minor effect on properties of t

32、he material.Low Carbon. Steel with approximately 6 to 25 points of carbon (0.06%0.25%)are rated as low carbon steels and are rarely hardened by heat treatment because the low carbon content permits so little formation of hard magnesite that the process is relatively ineffective. Enormous tonnages of

33、 these low carbon steels are processed in such structural shapes as sheet, strip,rod,plate,pipe,and wire. A large portion of the material is cold worked in its final processing to improve its hardness, strength, and surface-finish qualities.the grades containing 20 points or less of carbon are susce

34、ptible to considerable plastic flow and are frequently used as deep-drawn products or may be used as a ductile core for casehardened material. The low lain carbon steels are reality brazed, welded, and forged.Medium Carbon. The medium carbon steels (0.25%0.5%)contain sufficient carbon that they may

35、be heat treated for desirable strength, hardness, machinability, or other properties. The hardness of plain carbon steels in this range cannot be increased sufficiently for the material to serve satisfactorily as cutting tools,but the load-carrying capacity of the steels can be raised considerably,

36、while still retaining sufficient ductility for good toughness. The majority of the steel is furnished in the hot-rolled condition and is often machined for final finishing. It can be welded,but is more difficult to join by this method than the low carbon steel because of structural changes caused by

37、 welding heat in localized areas. High Carbon. High carbon steel contains from 50 to 160 points of carbon (0.8%1.6%). This group of steels is classed as tool and die steel, in which hardness is the principal property desired. Because of the fast reaction time and resulting low hardenability, and its

38、 associated danger of distortion or cracking, it is seldom possible to develop fully of heat-treat-hardened plain carbon steel is low compared to that of alloy steels with the same strength, but, even so, carbon steel is frequently used because of its lower cost.ALLOY STEELSAlthough plain carbon ste

39、els work well for many uses and are the cheapest steels and therefore the most used, they cannot completely fulfill the requirements for some work. Individual or groups of properties can be improved by addition of various elements in the form of alloys. Even plain carbon steels are alloys of at leas

40、t iron, carbon, and manganese, but the term alloy steel refers to steels containing elements other than these in controlled quantities greater than impurity concentration or, in the case of manganese, greater than 1.5%.Alloys Affect Hardenability. Interest in hardenability is indirect. Hardenability

41、 is usually thought of most in connection with depth-hardening ability in a full hardening operation. However, with the isothermal transformation curves shifted to the right, the properties forging operations, the materially usually air cools. Any alloy generally shifts the transformation curves to

42、the right, which with air cooling results in finer pearlite than would be formed in a plain carbon steel. This finer pearlite has higher hardness and strength, which has an effect on machinability and may lower ductility.Weldability. The generally bad influence of alloys on weldability is a further

43、reflection of the influence on hardenability. With alloys present is a further reflection of the influence on hardenability. With alloys present during the rapid cooling taking place in the welding area, hard, nonductile structures are formed in the steel and frequently lead to cracking and distorti

44、on.Grain Size and Toughness. Nickel in particular has a very beneficial effect by retarding grain growth in the austenite range. As with hardenability, it is the secondary effects of grain refinement that are noted in properties. A finer grain structure may actually have less hardenability, but it h

45、as its most pronounced effect on toughness; for two steels with equivalent in the chart as improved toughness. This improved toughness, however, may be detrimental to machinability. Corrosion Resistance. Most pure metals have relatively good corrosion resistance, which is generally lowered by impuri

46、ties or small amounts of intentional alloys. In steel, carbon in particular lowers the corrosion resistance very seriously. In small percentages, copper and phosphorus are beneficial in reducing corrosion. Nickel becomes effective in percentages of about %, and chromium is extremely effective in per

47、centages greater than %,which leads to a separate class of alloy steels called stainless steels. Many tool steels,while not designed for the purpose, are in effect stainless steels because of the high percentage of chromium present.LOW ALLOY STRUCTURAL STEELS Certain low alloy steels sold under vari

48、ous trade names have been developed to provide a low cost structural material with higher yield strengh than plain carbon steel. The addition of small amount of some alloying elements can raise the yield strength of hot-rolled sections without heat treatment to 30%40% greater than that of plain carb

49、on steels. Designing to higher working stresses may reduce the required section size by 25%30% at an increased cost of 15%50%,depending upon the amount and the kind of alloy.The low alloy structural steels are sold almost entirely in the form of hot -rolled structural shapes. These materials have go

50、od weldability, ductility, better impact strength than that of plain carbon steel, and good corrosion resistance, particularly to atmospheric exposure. Many building codes are based on the more conservative use of plain carbon steels, and the use of alloy structural steel often has no economic advan

51、tage in these cases.LOW ALLOY AISI STEELSImproved Properties at Higher Cost. The low alloy American iron and steel institute (AISI) steels are alloyed primarily for improved hardenability. They are more costly than plain carbon steels, and their use can generally be justified only when needed in the

52、 heat-treat-hardened and tempered condition. Compared to plain carbon steels, they can have 30%40% higher yield strength and 10%20% higher tensile strength. At equivalent tensile strengths and hardnesses, they can have 30%40% higher reduction of area and approximately twice the impact strength.Usual

53、ly Heat Treated. The low alloy AISI steels are those containing less than approximately 8% total alloying elements, although most commercially important steels contain less than 5%. The carbon content may very form very low to very high, but for most steels it is in the medium range that effective h

54、eat treatment may be employed for property improvement at minimum costs. The steels are used widely in automobile, machine tool, and aircraft construction, especially for the manufacture of moving parts that are subject to high stress and wear.STAINLESS STEELS Tonnage-wise, the most important of the higher alloy steels are a group of these steels have much better mechanical properties at high temperatures. This group was first called stainless steel. With the emphasis on high temperature use, they are frequently referr