凿岩钎具钎尾的热处理毕业论文

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凿岩钎具钎尾的热处理毕业论文
目 录
前言.....................................................2
凿岩钎具钎尾的热处理工艺探索............................. 3
1、钎尾的工作环境和失效形式分析...........................4
1.1 钎尾断裂..........................................4
1.2 尾部端面破损......................................4
1.3 螺纹磨损..........................................4
1.4 钎耳破断..........................................4
2、钎尾的服役条件.........................................4
3、钎尾的材料及合金元素的作用.............................5
4、En40B常用钎尾的热处理工艺分析..........................6
4.1 去应力退火.........................................6
4.2 渗碳直接淬火......................................7
4.2.1 渗碳.............................................7
4.2.2 直接淬火.........................................12
4.2.3 渗碳空淬后常见的缺陷及对策.......................13
4.3 低温回火.......................................... 16
4.4时效处理........................................... 16
5、En40B常用钎尾的新的热处理工艺...........................17
5.1 渗碳................................................17
5.2 等温淬火............................................17
结论........................................................19
参考文献....................................................21

凿岩钎具钎尾的热处理工艺探索
陈 梅
摘要：本文通过对钎具钎尾的工作环境、失效形式及钎尾用钢中所含的元素的种类和含量的分析；对凿岩钎具中材料为En40B的钎尾的热处理工艺进行分析探索；研究了钎尾在实际使用中的性能要求、实际生产中设备的先进性、金相组织、硬度实验；提出了针对En40B钎尾的热处理工艺中所涉及的问题；采用了对钎尾在工艺过程中各工序的温度高低、保温时间长短和可控气氛多用炉中碳浓度的高低的比较及所产生缺陷的分析，从而进-步提出了关于凿岩钎具钎尾的新的热处理工艺：渗碳-高温回火-等温淬火-低温回火；解决了En40B钎尾的热处理工艺参数选定原因和公司工艺中常存在的缺陷问题；达到了对En40B钎尾热处理工艺的理解，以及利用新工艺提高钎尾的使用性能的目的。
关键词：材料、钎尾、热处理
Decomposition of Austenite
The austenite to pearlite transformation is essentially the decomposition of austenite into almost pure ferrite and cementite .
At the equilibrium temperature，the transformation is impossible， since the free energy of the original austenite is equal to that of the final product ， pearlite .
The transformation can only start at a certain undercooling when the free energy of the ferrite carbide mixture (pearlite) is lower than that of austenite .
The lower the transformation temperature ，the higher the degree of undercooling and the greater the difference in free energies and the transformation proceeds at a higher rat .
In the pearlite transformation ， the new phases sharply differ in their composition from the initial phase ; they are ferrite which is almost free of carbon ， and cementite which contains6.67percent carbon. For that reason the austenite to pearlite transformation is accompanied with the diffusion ， redistribution of carbon . The rate of diffusion sharply diminishes with decreasing temperature ， therefore ， the transformation should be retarded at a greater undercooling .
Thus ，we have come to an important conclusion that undercooling (lowering the transformation temperature ) may have two opposite effects on the rate of transformation .
On one hand ， a lower temperature (greater undercooling ) gives a greater difference in free energies of austenite and pearlite ， thus accelerating the transformation ; on the other hand ， it diminishes the rate of carbon diffusion ，and thus slows down the transformation . The combined effect is that the rate of transformation first increases as undercooling is increased to a certain maximum and then decreases with further undercooling .
At 727℃(A1) and below 200℃ ，the rate of transformation is zero ， since at 727℃ the free energy difference is zero and below 200℃ the rate of carbon diffusion is zero (more strictly ，too low for the transformation to proceed ) .
As has been first indicated by I. L. Mirkin in 1939 and then developed by R. F.Mehl in 1941 ，the formation of pearlite is the process of nucleation of pearlite and growth of pearlite crystals .
Therefore ，the different rate of the pearlite transformation at various degrees of undercooling is due to the fact that undercooling differently affects the rate of nucleation N and the rate of crystal growth G .A temperature A1 and below 200℃ ，both parameters of crystallization N and G are equal to zero and have a maximum at an undercooling of 150～200℃ .
It follows from the foregoing that as soon as the conditions are favorable ，I .e .austenite is undercooled below A1 ，the diffusion of carbon is not zero ，centers of crystallization appear ，which give rise to crystals .this process occurs with time and can be represented in the form of so called kinetic curve of transformation ，which shows the quantity of pearlite that has formed during the time elapsed from the beginning of the transformation .
The initial stage is characterized by a very low rate of transformation ; this is what is called the incubation period . The rate of transformation increases with the progress in the transformation . Its maximum approximately corresponds to the moment when rougly 50 percent of austenite has transformed into pearlite .The rate of transformation then diminishes and finally stops .
The rate of transformation depends on undercooling the transformation proceeds slowly ， since N or G are low ;in the former case ， owing to a low difference in free energies ， and in the kinetic curves have sharp peaks ， and the transformation is finished in a short time interval .
At a high temperature (slight undercooling ) ，the transformation proceeds slowly and the time of the incubation period and the time of the transformation proper are long .At a lower temperature of the transformation ， i.e. a deeper undercooling ， the rate of transformation is greater ， and the time of the incubation period and of the transformation is shorter .
Having determined the time of the beginning of austenite to pearlite transformation (incubation period ) and the time of the end of transformation at various degrees of undercooling ，we can construct a diagram in which the left hand curve determines the time of the beginning of the transformation ， i.e. the time during which austenite still exists in the undercooled state ，and the section from the axis of ordinates to the curve is the measure of its stability . This section is shortest at a temperature of 500～600℃ ， i.e. the transformation begins in shortest time at that temperature .
The right hand curve shows the time needed to complete the transformation at a given degree of undercooling . This time is the shortest at the same temperature (500～600℃) .Note that the abscissa of the diagram is logarithmic .This is done for more convenience ，since the rates of formation of pearlite appreciably differ (thousands of seconds near the critical point A1 and only one or two seconds at the bend of the curve ) .
The horizontal line below the curves in the diagram determines the temperature of the diffusionless martensite transformation. The martensite transformation occurs by a different mechanism and will be discussed later.
Diagrams of the type we discussed are usually called TTT diagrams (time temperature transformation)， or C curves， owing to the specific shape of the curves. The structure and properties of the products of austenite decomposition depend on the temperature at which the transformation has taken places.
At high temperature ， i.e. low degrees of undercooling ，a coarse grained mixture of ferrite and cementite is formed which is easily distinguished in the microscope .This structure is called pearlite .
At lower temperatures ， and therefore ，greater degrees of undercooling ， more disperse and harder products are formed .The pearlite structure of this finer type is called sorbite.
At still lower temperature (near the end of the C curve )， the transformation products are even more disperse ，so that the lamellar structure of the ferrite cementite mixture is only distinguishable in the electron microscope .This structure is called troostite .
Thus ， pearlite ， sorbite and troostite are the structures of the same nature (ferritecementite) but a different dispersity of ferrite and cementite .
Pearlite structure may be of two types: granular (in which cementite is present in the form of grains ) or lamellar (with cementite platelets).
Homogeneous austenite always transforms into lamellar pearlite . Therefore ， heating to high temperature sets up favorable conditions for the formation of a more homogeneous structure and thus promotes the appearance of lamellar structures . Inhomogeneous austenite produces granular pearlite at all degrees of undercooling， therefore， heating to a low temperature (below AC3 for hypereutectoid steels) results in the formation of granular pearlite on cooling .The formation of granular cementite is probably promoted by the presence of undissolved particles in austenite ， which serve as additional crystallization nuclei .