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外文資料
Mechanical face work quality
The machine parts destruction, always starts generally from the surface layer.The product performance, in particular its reliability and the durability, are decided to a great extent by the components surface layer quality.Studies the machine-finishing surface quality the goal is in order to grasp in the machine-finishing each craft factor to process the surface quality influence the rule, in order to utilize these rules to control the processing process, finally achieved the improvement surface quality, enhances the product operational performance the goal.
Just processed between the friction vice-two faying surface, the initial stage in the surface roughness peak department contact, the actual contacted area far was only smaller than the theory contacted area, in the peak department which contacted mutually has the extremely big unit stress, caused the actual contacted area place to have the plastic deformation, the elastic deformation and the peak department between shearing failure, caused the serious attrition.
The components wear may divide into three stages generally, the initial period attrition stage, the normal wear stage and the fierce attrition stage.
The surface roughness the influence which wears to the components surface is very big.Said generally the surface roughness value is smaller, its wearability is better.But the surface roughness value too is small, the lubricating oil not easy to store up, between the contact face easy to have the molecular cementation, wears instead increases.Therefore, contact face roughness has a best value, its value and components working condition related, when work load enlarge, the initial period wearing capacity increases, the surface roughness best value also enlarges.
The processing surface hammer-harden makes the friction subtabulation surface layer metal the microhardness to enhance, therefore may cause the wear resistant enhancement generally.But is not the hammer-harden degree will be higher, the resistance to wear is higher, this is because the excessive hammer-harden will cause the metal organization to be excessively loose, even will appear the crack and surface layer metal flaking, will cause the resistance to wear to drop.
After metal the fatigue failure which is produced the alternating load function often occurs in the components surface and under the superficial chilling level, therefore the components surface quality is very big to the fatigue strength influence.
Under the alternating load function, the surface roughness saddle spot easy to set up stresses the centralism, produces the fatigue cracking.The surface roughness value is bigger, the superficial grain mark is deeper, the grain bottom radius is smaller, anti-fatigue failure bottom ability is worse.
The residue stress is very big to the components fatigue strength influence.The surface layer remaining tensile stress will make the fatigue cracking to expand, acceleration fatigue failure; But the surface layer residual stress can prevent the fatigue cracking the expansion, delays the fatigue failure the production
The surface chilling is accompanied by the residual stress generally the production, may prevent the crack has and prevents to have the crack expansion, to enhances the fatigue strength to be advantageous.
The components corrosion resistance is decided to a great extent by the surface roughness.The surface roughness value is bigger, then in the saddle accumulates the corrodent to be more.The corrosion resistance is worse.
The surface layer remaining tensile stress can have the stress corrosion dehiscence, reduces the components the resistance to wear, but the remaining compressed stress can prevent the stress corrosion dehiscence.
The surface roughness value size will affect the mating surface the coordinate quality.Regarding the gap coordination, roughness value congress caused the attrition enlarge, the gap to increase, destroyed the request coordinate nature.Regarding the full coordination, in the assembly process a part of superficial raised peak has been pushed evenly, the actual full quantity has reduced, reduced the coordination joint strength.
The cutting tool is opposite when makes the feed motion in the work piece, in the processing surface has left behind the layer of cutting residual area, when its shape the cutting tool geometry shape duplicate reflects.Reduces to feed quantity, the host angle of deflection, the vice-angle of deflection as well as increases the knife point circular arc radius, may reduce the residual area the altitude.
In addition, suitably increases the cutting tool the front rake reduces when the cutting the plastic deformation degree, the production which chooses the lubricant reasonably and improves the cutting tool sharpening quality to reduce when cutting the plastic deformation and the suppression built-up edge, the scale punctures, also is reduces the surface roughness value the effective action.
When processing plastic material, has had the plastic deformation by the cutting tool to the metal extrusion, adds avulsion function which the cutting tool force scrap and the work piece separate, causes the surface roughness value enlarge.The work piece toughness of material is better, the metal plastic deformation is bigger, the processing surface is rougher.
When processing brittle material, its scrap assumes garrulously granular, but leaves behind many hemp spots as a result of the scrap disintegration in the processing surface, causes the surface roughness.
When direct positive machining the surface roughness forming process is same, abrasive machining surface roughness formation also when decides by the geometry factor and the superficial metal plastic deformation.
The influence grinding surface roughness primary factor includes:
1、Grinding wheel granularity
2、Grinding wheel degree of hardness
3、Grinding wheel conditioning
4、Grinding speed
5、The grinding radial feed quantity and the light rub the number of times
6、Work piece circular feed speed and axial feed quantity
7、Cooling lubricant
In machining, work piece because receives the cutting force and cuts the hot function, causes the surface layer metal the physical mechanical property to have the change, the most main change is the surface layer metal microhardness change, the microstructure change and the residual stress production.Because time abrasive machining produces the plastic deformation and cuts hotly the knife edge cutting when is more serious than, thus the abrasive machining postprocessing surface layer above three physics mechanical property change can be very big.
In because the machine-finishing process the cutting action of force produces the plastic deformation, causes the crystal lattice distortion, the distortion, intergranular has the cutting slipping, the crystal grain is elongated with the fibrosis, even stave, these all can cause the surface layer metal degree of hardness and the intensity enhances, this kind of phenomenon is called the hammer-harden (or is called strengthening).The surface layer metal strengthens the result, can increase the flow of metal the resistance, reduces the metal the plasticity, the metal physical property also can change.
Is been at the high energy position by the hammer-harden metal the non-steady state, as soon as only then has the possibility, the metal non-steady state must to the quite stable condition transformation, this kind of phenomenon is called the attenuation.The attenuated function size is decided by the temperature height, the temperature duration length and the strengthened degree size.Because the metal simultaneously receives the strength and the hot function in the machine-finishing process, therefore, after the processing the surface layer metal final nature is decided by strengthened and the attenuated combined action result.
The evaluation hammer-harden target has three items, namely surface layer metal microhardness HV, hardened level depth h and hardenability N.
The cutting edge obtuse radius increases, strengthens to the surface layer metal extrusion function, the plastic deformation intensifies, causes the chilling enhancement.After the cutting tool the knife surface attrition increases, the latter knife surface with is processed superficial the friction aggravating, the plastic deformation increases, causes the chilling enhancement.
The cutting edge obtuse radius increases, strengthens to the surface layer metal extrusion function, the plastic deformation intensifies, causes the chilling enhancement.After the cutting tool the knife surface attrition increases, the latter knife surface with is processed superficial the friction aggravating, the plastic deformation increases, causes the chilling enhancement.
When is rubbed the work piece surface layer temperature to achieve when above transformation temperature, the surface layer metal has the microstructure change, causes the surface layer metal intensity and degree of hardness reduces, and is accompanied by the residual stress production, even appears the microscopic crack, this kind of phenomenon is called the grinding burn.
If the grinding area temperature has not surpassed the chilled steel the transformation temperature, but has surpassed the martensite the transition temperature, the work piece surface layer metal tempered martensite organization transforms degree of hardness low tempering organization (sorbite or troostite), this kind of burn is called the tempering burn.
If the grinding area temperature has surpassed the transformation temperature, in addition the refrigerant sharp quenching function, the surface layer metal has two quenching, causes the surface layer metal to appear two quenching martensite organization, its solidity ratio original tempered martensite high, in its lower level, because cools slowly, appeared the solidity ratio original tempered martensite low tempering organization (sorbite or troostite), this kind of burn is called the quenching burn.
If the grinding area temperature has surpassed the transformation temperature, but the grinding region does not have the refrigerant to enter, the surface layer metal will have the annealing organization, the skin hardness suddenly drops, this kind of burn will be called the annealing burn.
The grinding heat is creates the grinding burn the root, therefore improvement grinding burn by two ways: One is reduces the grinding to produce hotly as far as possible; Two, the improvement cooling condition, causes to have the quantity of heat as far as possible little to spread to the work piece.
1、Chooses the grinding wheel correctly
2、Reasonable choice cutting specifications
3、Improvement cooling condition
4、When cutting has the plastic deformation occurrence in the processing surface conductive sea, causes the superficial metal the specific volume enlarge
Because the plastic deformation only produces in the surface layer metal, but the surface layer metal specific volume increases, the volume expansion, needs to receive inevitably with its connected in level metal impediment, therefore on has had the residual stress in the superficial conductive sea, but has the remaining tensile stress in level metal. In the machining, the cutting district conference has the massive cutting hot production, the different microstructure has the different density, also has the different specific volume, if the surface layer metal has had the microstructure change, the surface layer metal specific volume change needs to receive inevitably with it connected substrate metal hindrance, thus has the residual stress production. La perfection de composants charge le choix final extérieur de méthode de transformation de méthode de travail est très importante, parce que la méthode de travail finale reste derrière dans ce visage actif que l'effort résiduel directement affectera la pièce de machine l'exécution opérationnelle.La perfection bien choisie de composants charge la méthode de transformation finale extérieure de méthode de travail, doit considérer ceci des composants que la perfection charge extérieur la condition de travail concrète et la forme possible de destruction.
中文譯文
機(jī)械表面加工質(zhì)量
機(jī)械零件的破壞，一般總是從表面層開始的。產(chǎn)品的性能，尤其是它的可靠性和耐久性，在很大程度上取決于零件表面層的質(zhì)量。研究機(jī)械加工表面質(zhì)量的目的就是為了掌握機(jī)械加工中各種工藝因素對(duì)加工表面質(zhì)量影響的規(guī)律，以便運(yùn)用這些規(guī)律來控制加工過程，最終達(dá)到改善表面質(zhì)量、提高產(chǎn)品使用性能的目的。
一個(gè)剛加工好的摩擦副的兩個(gè)接觸表面之間，最初階段只在表面粗糙的的峰部接觸，實(shí)際接觸面積遠(yuǎn)小于理論接觸面積，在相互接觸的峰部有非常大的單位應(yīng)力，使實(shí)際接觸面積處產(chǎn)生塑性變形、彈性變形和峰部之間的剪切破壞，引起嚴(yán)重磨損。
零件磨損一般可分為三個(gè)階段，初期磨損階段、正常磨損階段和劇烈磨損階段。
表面粗糙度對(duì)零件表面磨損的影響很大。一般說表面粗糙度值愈小，其磨損性愈好。但表面粗糙度值太小，潤(rùn)滑油不易儲(chǔ)存，接觸面之間容易發(fā)生分子粘接，磨損反而增加。因此，接觸面的粗糙度有一個(gè)最佳值，其值與零件的工作情況有關(guān)，工作載荷加大時(shí)，初期磨損量增大，表面粗糙度最佳值也加大。
加工表面的冷作硬化使摩擦副表面層金屬的顯微硬度提高，故一般可使耐磨性提高。但也不是冷作硬化程度愈高，耐磨性就愈高，這是因?yàn)檫^分的冷作硬化將引起金屬組織過度疏松，甚至出現(xiàn)裂紋和表層金屬的剝落，使耐磨性下降。
金屬受交變載荷作用后產(chǎn)生的疲勞破壞往往發(fā)生在零件表面和表面冷硬層下面，因此零件的表面質(zhì)量對(duì)疲勞強(qiáng)度影響很大。
在交變載荷作用下，表面粗糙度的凹谷部位容易引起應(yīng)力集中，產(chǎn)生疲勞裂紋。表面粗糙度值愈大，表面的紋痕愈深，紋底半徑愈小，抗疲勞破壞底能力就愈差。
余應(yīng)力對(duì)零件疲勞強(qiáng)度的影響很大。表面層殘余拉應(yīng)力將使疲勞裂紋擴(kuò)大，加速疲勞破壞；而表面層殘余應(yīng)力能夠阻止疲勞裂紋的擴(kuò)展，延緩疲勞破壞的產(chǎn)生
表面冷硬一般伴有殘余應(yīng)力的產(chǎn)生，可以防止裂紋產(chǎn)生并阻止已有裂紋的擴(kuò)展，對(duì)提高疲勞強(qiáng)度有利。
零件的耐蝕性在很大程度上取決于表面粗糙度。表面粗糙度值愈大，則凹谷中聚積腐蝕性物質(zhì)就愈多?？刮g性就愈差。
表面層的殘余拉應(yīng)力會(huì)產(chǎn)生應(yīng)力腐蝕開裂，降低零件的耐磨性，而殘余壓應(yīng)力則能防止應(yīng)力腐蝕開裂。
表面粗糙度值的大小將影響配合表面的配合質(zhì)量。對(duì)于間隙配合，粗糙度值大會(huì)使磨損加大，間隙增大，破壞了要求的配合性質(zhì)。對(duì)于過盈配合，裝配過程中一部分表面凸峰被擠平，實(shí)際過盈量減小，降低了配合件間的連接強(qiáng)度。
刀具相對(duì)于工件作進(jìn)給運(yùn)動(dòng)時(shí)，在加工表面留下了切削層殘留面積，其形狀時(shí)刀具幾何形狀的復(fù)映。減小進(jìn)給量、主偏角、副偏角以及增大刀尖圓弧半徑，均可減小殘留面積的高度。
此外，適當(dāng)增大刀具的前角以減小切削時(shí)的塑性變形程度，合理選擇潤(rùn)滑液和提高刀具刃磨質(zhì)量以減小切削時(shí)的塑性變形和抑制刀瘤、鱗刺的生成，也是減小表面粗糙度值的有效措施。
加工塑性材料時(shí)，由刀具對(duì)金屬的擠壓產(chǎn)生了塑性變形，加之刀具迫使切屑與工件分離的撕裂作用，使表面粗糙度值加大。工件材料韌性愈好，金屬的塑性變形愈大，加工表面就愈粗糙。
加工脆性材料時(shí)，其切屑呈碎粒狀，由于切屑的崩碎而在加工表面留下許多麻點(diǎn)，使表面粗糙。
正像切削加工時(shí)表面粗糙度的形成過程一樣，磨削加工表面粗糙度的形成也時(shí)由幾何因素和表面金屬的塑性變形來決定的。
影響磨削表面粗糙的主要因素有：
1、砂輪的粒度
2、砂輪的硬度
3、砂輪的修整
4、磨削速度
5、磨削徑向進(jìn)給量與光磨次數(shù)
6、工件圓周進(jìn)給速度與軸向進(jìn)給量
7、冷卻潤(rùn)滑液
在切削加工中，工件由于受到切削力和切削熱的作用，使表面層金屬的物理機(jī)械性能產(chǎn)生變化，最主要的變化是表面層金屬顯微硬度的變化、金相組織的變化和殘余應(yīng)力的產(chǎn)生。由于磨削加工時(shí)所產(chǎn)生的塑性變形和切削熱比刀刃切削時(shí)更嚴(yán)重，因而磨削加工后加工表面層上述三項(xiàng)物理機(jī)械性能的變化會(huì)很大。
機(jī)械加工過程中因切削力作用產(chǎn)生的塑性變形，使晶格扭曲、畸變，晶粒間產(chǎn)生剪切滑移,晶粒被拉長(zhǎng)和纖維化，甚至破碎，這些都會(huì)使表面層金屬的硬度和強(qiáng)度提高，這種現(xiàn)象稱為冷作硬化(或稱為強(qiáng)化)。表面層金屬?gòu)?qiáng)化的結(jié)果，會(huì)增大金屬變形的阻力，減小金屬的塑性，金屬的物理性質(zhì)也會(huì)發(fā)生變化。
被冷作硬化的金屬處于高能位的不穩(wěn)定狀態(tài)，只有一有可能，金屬的不穩(wěn)定狀態(tài)就要向比較穩(wěn)定的狀態(tài)轉(zhuǎn)化，這種現(xiàn)象稱為弱化。弱化作用的大小取決于溫度的高低、溫度持續(xù)時(shí)間的長(zhǎng)短和強(qiáng)化程度的大小。由于金屬在機(jī)械加工過程中同時(shí)受到力和熱的作用，因此，加工后表層金屬的最后性質(zhì)取決于強(qiáng)化和弱化綜合作用的結(jié)果。
評(píng)定冷作硬化的指標(biāo)有三項(xiàng)，即表層金屬的顯微硬度HV、硬化層深度h和硬化程度N。
切削刃鈍圓半徑增大，對(duì)表層金屬的擠壓作用增強(qiáng)，塑性變形加劇，導(dǎo)致冷硬增強(qiáng)。刀具后刀面磨損增大，后刀面與被加工表面的摩擦加劇，塑性變形增大，導(dǎo)致冷硬增強(qiáng)。
切削速度增大，刀具與工件的作用時(shí)間縮短，使塑性變形擴(kuò)展深度減小，冷硬層深度減小。切削速度增大后，切削熱在工件表面層上的作用時(shí)間也縮短樂，將使冷硬程度增加。進(jìn)給量增大，切削力也增大，表層金屬的塑性變形加劇，冷硬作用加強(qiáng)。工件材料的塑性愈大，冷硬現(xiàn)象就愈嚴(yán)重。 當(dāng)切削熱使被加工表面的溫度超過相變溫度后，表層金屬的金相組織將會(huì)發(fā)生變化。
當(dāng)被磨工件表面層溫度達(dá)到相變溫度以上時(shí)，表層金屬發(fā)生金相組織的變化，使表層金屬?gòu)?qiáng)度和硬度降低，并伴有殘余應(yīng)力產(chǎn)生，甚至出現(xiàn)微觀裂紋，這種現(xiàn)象稱為磨削燒傷。在磨削淬火鋼時(shí)，可能產(chǎn)生以下三種燒傷：
如果磨削區(qū)的溫度未超過淬火鋼的相變溫度，但已超過馬氏體的轉(zhuǎn)變溫度，工件表層金屬的回火馬氏體組織將轉(zhuǎn)變成硬度較低的回火組織(索氏體或托氏體)，這種燒傷稱為回火燒傷。
如果磨削區(qū)溫度超過了相變溫度，再加上冷卻液的急冷作用，表層金屬發(fā)生二次淬火，使表層金屬出現(xiàn)二次淬火馬氏體組織，其硬度比原來的回火馬氏體的高，在它的下層，因冷卻較慢，出現(xiàn)了硬度比原先的回火馬氏體低的回火組織(索氏體或托氏體)，這種燒傷稱為淬火燒傷。
如果磨削區(qū)溫度超過了相變溫度，而磨削區(qū)域又無冷卻液進(jìn)入，表層金屬將產(chǎn)生退火組織，表面硬度將急劇下降，這種燒傷稱為退火燒傷。
磨削熱是造成磨削燒傷的根源，故改善磨削燒傷由兩個(gè)途徑：一是盡可能地減少磨削熱地產(chǎn)生；二是改善冷卻條件，盡量使產(chǎn)生地?zé)崃可賯魅牍ぜ?
1、正確選擇砂輪
2、合理選擇切削用量
3、改善冷卻條件
4、切削時(shí)在加工表面金屬層內(nèi)有塑性變形發(fā)生，使表面金屬的比容加大
由于塑性變形只在表層金屬中產(chǎn)生，而表層金屬的比容增大，體積膨脹，不可避免地要受到與它相連的里層金屬的阻止，因此就在表面金屬層產(chǎn)生了殘余應(yīng)力，而在里層金屬中產(chǎn)生殘余拉應(yīng)力。切削加工中，切削區(qū)會(huì)有大量的切削熱產(chǎn)生,不同金相組織具有不同的密度，亦具有不同的比容,如果表面層金屬產(chǎn)生了金相組織的變化，表層金屬比容的變化必然要受到與之相連的基體金屬的阻礙，因而就有殘余應(yīng)力產(chǎn)生。 零件主要工作表面最終工序加工方法的選擇至關(guān)重要，因?yàn)樽罱K工序在該工作表面留下的殘余應(yīng)力將直接影響機(jī)器零件的使用性能。選擇零件主要工作表面最終工序加工方法，須考慮該零件主要工作表面的具體工作條件和可能的破壞形式。