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英文原文
Surface quality
Surface quality is anther aspect of machining accuracy.it consists of the surface finish and the physico-mechanical state of the surface layer. It affects the proper functing and serveice life of the psrts.
1. Surface finish
Machining accuracy is expressed quantitatively in machining error. Surface finish is expressed quantitatively in surface roughness. The surface roughness on a machined workpiece surface is caused primarily by the geometrical factors(cutting tool geometry and cutting feed),the plastic deformation of surface material, as the vibration of the MFTW system during machining.
The surface roughness of a workpiece after machining depends upon many technological factors, such as the workpiece material,tool material, tool geometry, cutting condition (cutting speed and feed ), rigidity of the MFTW system, cooling conditonsin machining process, and so on. The surface roghness is a kind of microgemetrical deviation or micro-irregularity of workpiece surface. It appears on all machined workpiece surface no matter how smooth they look to the eye.In regard to the criteria for eveluating surface roghness, refer to other textbooks, ISO, and other standards and references.
The surface roghness of machined parts influences their performance to a great extent. In order to ensure product quality,to improve its service life, and to reduce its production cost, the surface roghness of machine parts has to be specified accurately in design and carefully controlled in manufacturing.
2.Physico-Mechanical Stae of Surface Layer
In machining process, the metal nearest the workpiece surface deforms plastically. This is due to the intrusion of the cutting tool rake to the workpiece surface material, the friction between the tool relife and the machined surface, and the effect of the tool nose radius. This results in the surface layer on the workpiece in quite a different way from the internal material of the workpiece after machining .until now, the evaluation of the physico-mechanical state of the surface layer is still in the experimental investigation stage. Complete standards for the evaluation do not exist .however, it is known that the variation of the material on the surface layer from the original metal are caused by cold hardening ,metallographical change, and residual stress.
The cold hardening of the surface layer material is caused by plastic deformation of the material, resulting in the increase of its microhardness. The parameters which characterize the cold hardening are shown below:
The hardened depth h , which is the depth of the surface layer, deformed plastically in the machining process.
The microhardness H. The degree of cold hardening N , which is the ratio of the microhardness , increases the surface layer to the microhardness of the original metal, i.e,
where stands for the microhardness of the original metal.
A large part of the energy consumed in machining is transfomed into heat, which results in the inctease of the temperature in the cutting or grinding area, especially on the workpiece surface being machined. In general cutting processes, most of the hear generated is carried away by the temperature of the workpiece surface is not very high. However, in the machining processes consuming a very large amount of energy, for instance as in grinding , the temperature of the workpiece surface may reach or exceed the critical point or phase change of workpiece material. This codition causes the metallographical variation of the surface material.
The residual stress is generated in the surface layer after machining. The causes of the generation of the residual stress are: (i) The material of the workpiece surface expands while it is heated by the cutting or grinding heat, and it contracts when it is cooled. The expansion and contraction are resisted by the internal material of the workpiece which results in the residual tensilsstress in the surface layer after machining; (ii) The surface material plastically deforms under the extrusion and friction of the tool, while the internal material close to the surface layer elastically defoms. After machining, the recovery of the elastic deformation of the internal material is restricted by the surface material which has deformed plastically. This results in the residual stress, udually compressive residual stress, in the surface layer;(iii) The metallographical variation of the surface layer leads to its volume change, either expansion or contraction, which is restricted by the internal material and result in the residual stress(compressive or tensile)in the surface layer.
3.Infuluence of Surface Puality on the Performance of Machined Parts
A field surface of a machined part caused by its wear, fatigue or corrosion, etc. ,starts from the part surface in most case. The surface quality of a machine part greatly influences its performance, such as the fitting between parts, wear resistance, corrosion resistance, and fatigue strenght.
4. Influence on Fitting between Parts
The surface roghness influences directly the fitting condition between parts. For the clearance fit, the existence of the micro-irregularities on part surface leads to a rapid initial wear. Thus the clearance increases between the parts which cause the deterioration of the fitting condition.
As to the interference fit, the strengh of fit depends upon the surface roughness of the parts. When one part is press-fitted into another, the ridges of the micro-irregularities are extruded: This reduces the actual interference and thus the strength of fit. The actual interference can be calculated by the following equation:
Where e—actual interference
— maximum height of irregularities of shaft, repectively,
—maximum height of irregularities of hole, repectively,
—dimeter of shaft,
—dimeter of hole
The surface roughness and surface hardening influence greatly the wear resistance of part surfaces. When dry friction exists between two fitting surface, only the ridges of the irregularities on both surface are in contact with each other at the initial stage. The pressure between the two mating parts is concentrated on these small areas.
For example, the actual contacting area for turned or milled surfaces is only 15~20% of the total area, and for the finely ground surfaces, 30~50% . Due to the high unit pressure, the irregularities on the part surfaces defrom elastically and plastically and portions are sheared because of the relative movement of the surfaces. These fallen particles are scattered which intensify the wear between the contacting surfaces.
The situation of the wet friction is more complicated. At the initial stage of the wet friction the lubricant films are pierced through by the ridges of the roughness to from a dry friction between two fitting surfaces. The intensive initial wear changes the proper fit of the mating parts. However ,as the wear gradually increases ,the until pressure between the surfaces reduces, and the wear slows down. When a certain point is reached,the wear is intensified again.
A resuction of the surface roughness can increasa the wear resistance of part surface. Howere, the relationship between them is not linear. It has been proved by experiments that an optimal value for the surface roghness exists under a given condition with which the minimum wear can be expected. If the fitting surface are too smooth, they will be in close contact with each other which leads to a larger affinity between the molecules of the surface. This will result in an intensive wear of surface.
The hardening of the surface layer can greatly increase the wear resistance of part surface. However, an excessive increase of the microhardness may cause the peeling of the hardened layer. Therefore , caution should be exercised when applying microhardening.
5.Influence on Corrosion Resistance
The surface roghness greatly influences the corrosion resistance of a part surface. Usually the corrodion subtances are gathered at the valleys of the surface micro-irregularities which will attack the part surface. The deeper and sharply defined the valleys between the ridges of microrregularities, the more destructive the effect of corrosion directed into the depth of the metal.
The hardening of the surface layer and the existence of the residual stress in the surface layer will reduce the corrision resistance of the part surface. This is because the structure of the surface layer after plastic deformation is in a non-equilibrium state, possessing higher energy. It is more prone to corrosion.
6.Influence on Fatigue Strengh
The destructive failure of metal parts under slternate loads starts from part surface or under a certain depth of the hardened suface. Consequently the fatigue strengh of a part depends upon the surface quality of the part to a great extent. Subject to periodically alternate load, the valleys between the ridges of surface micro-irregularities may become the points of internal stress concentration which may result in the failure of the part. The higher the type finish, the higher the fatigue strength of the part.
Cold hardening of the surface layer pevents the extension of existing microcracks. The generation of new microcracks on a part surface reduces the harmful effct of the surface roghness and the external defects of the part. This helps the increase of the fatigue strength. Howere , the excessive cold hadening of the surfce layer may intensify the extension of the microcracks. Under alternate load or high temperature, this decreases certain limits.
The influence of the residual stress in surface layer on the fatigue strength of a part depends on the direction and the magnitude of the residual stress. The compressive residual stress tends to colse the microcracks and thus its existence can greatly increase the fatigue strength of the part. On the other hand , the existence of the tensile residual stress will decrease the fatigue strength. However since the generation of the tensile residual stress is always accompanied with the hardening of the surface layer, this harmful effect is reduced.
It can be seen from the above that the hardening of the surface layer and the existence of the compressive residual stress are conductive to the increase of the fatigue strength of parts. Appropriate methods can be adopted to generate hardening and compressive residual stresses in the surface layer,such as the ball peening, the burnishing, ect. Some heat-treatment methods, such as carbonizing and nitriding, can also generate the compressive residual stress in the surface layer. In addition, some micro-finishing methods, such as polishing and vibrated finishing, ect. ,can be used to improve the surface finish, thus leading to the increase of the fatigue strength of parts.
中文譯文
表面質(zhì)量
表面質(zhì)量是機械加工精度的一方面，它包括表面粗糙度和表面層物理機械狀態(tài)。它影響了零件的使用性能和使用壽命。
1、表面光潔度
機械加工精度是用機械加工誤差來定量表示的，表面光潔度是用表面粗糙度來定量表示的。一個已加工表面的表面粗糙度，主要由幾何因素（刀具的幾何形狀和走刀量），材料表面的塑性變形，以及在機械加工過程中的MFTW系統(tǒng)的振動引起的。
工件加工的表面粗糙度取決于許多技術(shù)因素，例如：工件的材料、刀具的材料、刀具的幾何形狀、切削條件（切削速度和走刀量）、MFTW系統(tǒng)的剛性、機械加工過程中的冷卻條件，等等。表面粗糙度是工件表面的一種微觀幾何偏差或微觀不規(guī)則性。不論用肉眼看起來是多么地光滑，它都會在已加工工件表面出現(xiàn)。參考其他課本，ISO標準和其他標準文獻。
已加工零件的表面粗糙度在很大程度上影響它們的性能。為了確保零件質(zhì)量，保證它服務(wù)壽命和降低零件成本，機器零件的表面粗糙度必須在設(shè)計過程中精確的規(guī)定和加工制造過程中仔細控制。
2、表面層的物理機械狀態(tài)
在機械加工過程中，最靠近工件表面的金屬產(chǎn)生塑性變形。這是由于刀具前刀面對工件表面材料的擠壓。刀具后刀面與正加工表面之間的摩擦和刀尖半徑的影響。這就導致機械加工后工件表面層與其心部材料處于完全不同的狀態(tài)。直到現(xiàn)在，表層的物理機械狀態(tài)評價仍然處于試驗性的研究階段。完整標準的評價是不存在的。然而，眾所周知是從原始金屬發(fā)生表面層材料冷作硬化、金相組織變化和殘余應(yīng)力引起的各種變化。
表層材料的冷作硬化包括材料的塑性變形，導致它的微觀硬度的增加。表征冷作硬化的參數(shù)如下：
硬化深度h ,它是表層材料在機械加工過程中塑性變形的深度。
微觀硬度H，冷作硬化程度N是指表層微觀硬度的增量與原始金屬微觀硬度的比率，即：
式中表示原始金屬的微觀硬度。
機械加工過程中一部分的能量消耗用于熱量轉(zhuǎn)換，原因是切削或磨削區(qū)域的溫度升高，尤其是正在加工的工件表面。一般在切削過程中產(chǎn)生的大部分熱量被切屑帶走，因此工件表面的溫度不是很高。然而，在機械加工過程中也消耗了大量的能量，例如在磨削過程中，工件表面的溫度可能達到甚至超過工件材料相變的臨界點。這個因素導致了表面材料的金相組織變化。
加工后的表面產(chǎn)生殘余應(yīng)力，其產(chǎn)生殘余應(yīng)力的原因是：1、在切削或磨削時產(chǎn)生的熱量使得工件表面材料膨脹，冷卻時又會收縮。膨脹和收縮都會受到內(nèi)部材料的限制，從而導致在機械加工后表面產(chǎn)生殘余應(yīng)力。2、材料的表面在刀具的擠壓和摩擦下產(chǎn)生塑性變形，而緊挨著表面的內(nèi)部材料發(fā)生彈性變形。加工后，內(nèi)部材料的彈性變形的恢復又受到了已發(fā)生塑性變形的表面材料的限制。這就導致了表面的殘余應(yīng)力，通常產(chǎn)生的是表面層殘余壓應(yīng)力。表面層的各種金相組織的變化導致了材料體積的改變，不論是延伸還是收縮都會受到內(nèi)部材料的限制，并且導致了表面層殘余應(yīng)力（壓和拉）的產(chǎn)生。
3、表面質(zhì)量對已加工零件性能的影響
由于磨損、疲勞和腐蝕等原因引起的加工過的零件的現(xiàn)場失效，絕大多數(shù)情況都是從零件表面開始的。機器零件的表面質(zhì)量極大地影響了它的性能，例如零件之間的配合、抗腐蝕性和疲勞強度。
4、對零件之間配合的影響
表面粗糙度直接地影響了零件的配合情況。對于間隙配合、零件表面上的微觀不規(guī)則的存在導致了初期磨損的加速。從而使配對零件間的間隙增加同時引起了配合情況的惡化。
對于過盈配合、配合強度、取決于零件的表面粗糙度。當一個零件被壓進另一個零件時，微觀不規(guī)則的波峰受到擠壓；這就減小了實際過盈量，從而配合強度也會降低。實際的過盈量可以通過以下的公式計算出來：
式中：
e－實際過盈量
－代表軸的不規(guī)則性的最大高度；
－代表孔的不規(guī)則性的最大高度；
－軸的直徑；
－孔的直徑。
表面粗糙度和表面硬化程度極大地影響零件表面的耐磨性，當兩個相互配合表面存在干磨擦時，在初期階段只有兩個表面的微觀不規(guī)則的波峰相互接觸。兩個配合零件間的壓力就集中在這些小區(qū)域上，例如：車或銑的表面的實際接觸面積只占整個面積的15~20%，并且精密磨削表面也只占30~50%。由于在單位壓力很高的情況下，在零件表面的微觀不規(guī)則處，由于表面間的相對運動就會發(fā)生彈性變形、塑性變形，并且部分被剪切掉，這些掉下來的微粒被分散，從而加劇了接觸表面的磨損。濕摩擦的狀態(tài)是更復雜的，在濕摩擦的初期，兩個配合表面的潤滑油油膜被粗糙處的尖峰刺破，從而形成干摩擦，加劇了初期磨損，改變了兩個配合零件的正確配合。然而，隨著磨損的逐漸增加，表面間的比壓減小，并且磨損也會減慢，當磨損達到某一點時，磨損又會再次加劇。
表面粗糙度的降低能夠增大零件表面的抗磨性，但是，表面粗糙度和耐磨性之間的關(guān)系并不是線性的，通過實驗證明：在給定的條件下，對于表面粗糙度存在一個最優(yōu)值，使期望的磨損達到最小。如果配合表面太光滑，它們將此緊密接觸，從而導致在表面的分子之間產(chǎn)生一個較大的親合力。這將導致表面間有一個劇烈的磨損。
表面層的微觀硬化能極大的影響零件表面的抗磨性，然而一個過大的微觀硬化，可能造成硬化層的剝落。因此，微觀硬化時必須十分小心。
5、對抗腐蝕性的影響
表面粗糙度極大地影響了零件表面的耐腐蝕性，通常腐蝕物質(zhì)被積聚在微觀不平的低谷，它們將侵蝕零件表面。微觀不平度的波峰之間的波谷越深和越尖，對深度金屬的腐蝕效果越具有破壞性。
表面層硬化和在表面層的殘余應(yīng)力的存在，將會減小零件表面的耐腐蝕性，這是因為表面層的結(jié)構(gòu)在塑性變形后處于一個非平衡階段，具有很高的能量。因此，更易于腐蝕。
6、疲勞強度
金屬零件在交變載荷作用下的破壞性失效是從零件表面或者硬化層表面下的某一個深度開始的。因而，一個零件的疲勞強度在很大程度上取決于零件的表面質(zhì)量。受到周期性交變載荷的作用，微小不平整表面的波峰之間的波谷可能變成內(nèi)應(yīng)力的集中點，因此，可能引起零件的失效。表面光潔度的級別越高，零件的疲勞強度就越高。表面的冷作硬化阻止了微觀裂縫的擴大，在零件表面新的微觀裂縫的產(chǎn)生減少了零件的表面粗糙度和外部缺陷的有害影響。有助于提高疲勞強度，然而，表面層過度的冷作硬化可能加劇微觀裂縫的擴展。在交變載荷或高溫下，它減小了零件的疲勞強度。因此，冷作硬化應(yīng)該被控制在某一范圍內(nèi)。
表面層殘余應(yīng)力對零件疲勞強度的影響取決于殘余應(yīng)力的傾向和大小。殘余壓應(yīng)力傾向于縮小微觀裂縫，因此，它的存在極大地增加了零件的疲勞強度。相反，殘余拉應(yīng)力的存在將減小疲勞強度。盡管如此，由于殘余應(yīng)力的產(chǎn)生總是伴隨著表面層的硬化，所以拉應(yīng)力對疲勞強度的副面影響將被減小。
從以上可以看出，關(guān)于表面層硬化和殘余壓應(yīng)力的存在有助于增加零件的疲勞強度?？梢圆捎们‘?shù)姆椒ㄊ蛊洚a(chǎn)生表面層硬化和殘余應(yīng)力，例如：噴丸、壓力拋光等等。有些熱處理方法，例如：滲碳、氮化等等，也能在表面層上產(chǎn)生殘余壓應(yīng)力。還有一些微處理法，例如：拋光、振動精加工等等，也可以用來提高表面光潔度，從而導致零件疲勞強度的增加。