【文章內(nèi)容簡介】 除速度。 is removed. 9 Welding at a slower rate puts in more heat and heats a large volume of metal, so the cooling rate due to loss of heat to the base metal is decreased(減少 ). A preheat will do the same thing. For example, sae4150 steel, preheated to 600 or 800, can be welded readily（容易地） . When the flame or arc is taken away from the weld, the cooling rate is not so great, owing to the higher temperature of the surrounding metal and slower cooling results. Even the most rapid airhardening（風硬鋼） steels are weldable if preheated and welded at a slow rate. 低速焊接帶來了更多的熱量，這對金屬的大量體積進行了加熱，所以冷卻速度降低。預(yù)熱可以取得與之相當?shù)男Ч?。例如? 被預(yù)熱至 或 時可以很好的焊接。由于周圍金屬的較高溫度，當焊接弧移開焊接點后，冷卻速度不會太快，產(chǎn)生了低速冷卻的結(jié)果。即使是冷作硬化速度最快的金屬也可以通過預(yù)熱和慢速焊接達到良好的焊接效果。 Machinability 10 Machinability（機械加工性能） means several things. To production men it generally means being able to remove metal at the fastest rate, leave the best possible finish, and obtain the longest possible tool life. Machinability applies to（應(yīng)用于） the toolwork（ 工具，零件） bination. 可加工性 可加工性意味著幾件事情。對于加工者來說，它意味著可以快速的移除金屬，取得最好的加工效果，得到最長的刀具壽命?？杉庸ば允堑毒吆土慵慕Y(jié)合。 11 It is not determined by hardness（硬度） alone, but by the toughness（韌性） , microstructure, chemical position（成分） , and tendency（傾向） of a metal to harden under cold work. In the misleading expression “too hard to machine”, the word “hard” is usually meant to be synonymous（同義的） with “difficult”. Many times a material is actually too soft to machine readily. Softness and toughness may cause the metal to tear（撕裂） and flow ahead of the cutting tool rather than cut cleanly. Metal that are inherently（天性地，固有地） soft and tough are sometimes alloyed to improve their machinability at some sacrifice（犧牲） in ductility. Examples are use of lead（鉛） in brass（黃銅） and of sulfur（硫磺） in steel. 加工性不僅僅只由硬度決定，它還由韌性，微觀結(jié)構(gòu)，化學成分和在冷加工下金屬所呈現(xiàn)的硬化特性所決定。在容易混淆的表示“難加工”中，“ hard”與“ difficult”同義。許多時候，因為材料過軟而難于穩(wěn)定加工。材料柔軟性和韌性能夠產(chǎn)生金屬撕裂，使金屬在完成切削前流動至刀具前端。柔軟的金屬往往會被加入合金從而犧牲它的延展性來提高加工性能。如黃銅中加入鉛鋼中加入硫磺。 12 Machinability is a term used to indicate the relative（比較的） ease（不費力） with which a material can be machined by sharp cutting tools in operations such as turning（車） , drilling（鉆） , milling（銑） , broaching（拉削） , and reaming（鉸） . 機械加工性能是在指對工件材料使用刀具進行諸如車、鉆、銑、拉削、鉸加工時的難易程度。 13 In the machining of metal, the metal being cut, the cutting tool, the coolant, the process and type of machine tool（機床） , and the cutting 在對金屬進行加工時，被切削的金屬，切削刀具，冷卻液，使用的機床的種類，切削條件均影響著切削效果。改變?nèi)魏我环N均會產(chǎn)生conditions all influence the results. By changing any one of these factors, different results will be obtained. The criterion（標準） upon which the ratings（等級） listed are based(等級評定的標準 ) is the relative volume of various（不同種） materials that may be removed by turning under fixed conditions to produce an arbitrary（任意的） fixed amount of tool wear. 不同的切削效果。切削效果評定的準則是：車削時在固定的切削條件下產(chǎn)生一定量的刀具磨損時，被加工試件相應(yīng)的材料去除量。 Unit 3 Mechanical Properties of Materials 1 The material properties can be classified into three major headings: (i) Physical, (ii) Chemical, (iii) Mechanical. Physical properties 2 Density or specific gravity, moisture content, etc., can be classified under this category. Chemical properties 3 Many chemical properties e under this category. These include acidity or alkalinity, reactivity and corrosion. The most important of these is corrosion which can be explained in layman’s terms as the resistance of the material to decay while in continuous use in a particular atmosphere. Mechanical properties 4 Mechanical properties include the strength properties like tensile, pression, shear, torsion, impact, fatigue and creep. The tensile strength of a material is obtained by dividing the maximum load, which the specimen bears by the area of crosssection of the specimen. 5 This is a curve plotted between the stress along the Yaxis(ordinate) and the strain along the Xaxis(abscissa) in a tensile test. A material tends to change or changes its dimensions when it is loaded, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappear s. For many materials this occurs up to a certain value of the stress called the elastic limit. This is depicted by the straight line 材料特性主要分為三類： (i)物理特性，(ii)化學特性， (iii)力學性能。 物理特性 密度或特定的重力，濕度等都屬于此范疇。 化學特性 許多化學特性都歸入到這個范疇。其中包括酸性或堿性，活性和耐腐蝕性。而在這其中最重要的是耐腐蝕性，通俗的解釋是材料在特定大氣中長期使用時，抵抗腐蝕的能力。 力學特性 力學特性包括諸如拉伸，壓縮，剪切，扭轉(zhuǎn)，沖擊，疲勞和蠕變等強度特性。一種材料的拉伸強度由試件承載的最大載荷除以試件的橫截面積得到。 如圖所示為在拉伸試驗中沿著 X 軸 （橫軸）的應(yīng)變和沿著 Y 軸（縱軸）的應(yīng)力之間的關(guān)系曲線。材料在加載時，隨著載荷大小的變化，尺寸會發(fā)生改變。當卸載時，變形消失。對于許多材料來說，上述情況發(fā)生的應(yīng)力極限值稱為彈性極限。在應(yīng)力 應(yīng)變曲線中，直線關(guān)系和隨后的小小的彎曲描述了上述的加載和卸載。 relationship and a small deviation thereafter, in the stressstrain curve. 6 Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality. In this region, the metal obeys Hooke’s law, which states that the stress is proportional to strain in the elastic range of loading (the material pletely regains its original dimensions after the load is removed). In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the elastic limit. This may be attributed to the timelag in the regaining of the original dimensions of the material. This effect is very frequently noticed in some noferrous metals. 7 While iron and nickel exhibit clear ranges of elasticity, copper, zinc（鋅） , tin（錫） , etc, are found to be imperfectly elastic even at relatively（相當?shù)兀? low values of stresses. Actually the elastic limit is distinguishable（可區(qū)分的） from the proportionality（比例性） limit more clearly depending upon the sensitivity （ 靈敏 性） of the measuring ins