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机械毕业设计英文外文翻译90大尺寸塑料模具钢AISI.P20钢的淬火过程设计

机械毕业设计英文外文翻译90大尺寸塑料模具钢AISI.P20钢的淬火过程设计
机械毕业设计英文外文翻译90大尺寸塑料模具钢AISI.P20钢的淬火过程设计

翻译译文:

大尺寸塑料模具钢AISI.P20钢的淬火过程设计

大尺寸的AISI.P20钢,用于厚度不小于200mm的塑料模具钢,为获得一定的淬硬层,适当的热处理过程是十分关键的。本文就AISI.P20钢的不同热处理过程如油淬,直接水淬,水淬前预冷和自我回火,做了计算机模拟的数字调研,对数学模型热处理过程细节做了讨论,包括局部热传递的不同反应式,热物理性质,潜热,热传递系数和相变计算,结果显示;预冷水淬和自我回火,不仅能有效地防止淬裂,而且比油淬获得更深的淬硬层

关键词:计算机模拟,热处理过程,AISI.P20钢,塑料模具

1.前言

AISI.P20钢,塑料模具钢,通常淬硬前硬度值大约30~36HRC。P20钢应该有均匀的硬度,在同一截面上,硬度值最大不超过3HRC,这就要求共析铁素体尽可能少。AISI.P20钢:0.28%~0.40%C,0.20%~0.80%Si,0.60%~1.00%Mn,1.40~2.0%Cr,0.30%~0.553%Mo,<0. 03%S,<0.03%P。油淬时钢的最大厚度不应超过150mm。厚度超过150mm的大尺寸的钢,为获得一定的淬硬层,适当的热处理过程是非常关键的。然而热处理过程是包括热传递现象,冶金性,力学现象等复杂的过程。它很难或者说不可能用分析的方法来正确而有效地描述所有的物理现象。所以,在过去,热处理过程的建立只是根据经验,而缺乏科学依据。近年来,计算机模拟使其有了巨大的进步。进行温度与相度应力的耦合。3D非线性有限元素分析,相变动力的计算。对急剧转变边界条件的处理等。这些计算结果为引导制造业的计算机模拟,提供了一些基本法则。本文中,分析由计算机模拟的AISI.P20钢的不同热处理过程,提出预冷水淬和自我回火

2.热处理过程的数学模型

2.1热传递过程中,局部不同的反应:

大尺寸钢在热处理过程中的导热量,可以通过3D模型局部不同反应方程计算出来注:7

注: 8

t

T pc q z T z y T y x T ??=+????+????+????ρλλλχ)()()( (1) 边界条件和初始条件为:

00()(,,)s t T h T T n

T T x y z αλ=?-=-?=(2)(3)

λ是导热性,ρC 是比热容,q 是内热源

n T ??是垂直于表面的温度梯度,s T 是表面温度,αT 是淬火介质温度, n 是传热系数。边界条件与初始条件作为钢的表面温度的函数输入。

在分析瞬时温度时,3D 有限元分析法解传热方程用前一个反应式,当时间范围是离散的,应用后向差分格式。

[][]{}[]{}{}11(4)t t t K C T C T F t t -???+=+ ?????

这里的[]K 代表模具导热率,[]C 代表模具热容率,{}F 是供热介质。 热物理性质:

图1 在AISI P20钢上不同微观组织的热物理性质:(a )导热率(b )比热(c )密度(M=马氏体,B=贝氏体,P=珠光体,F=铁素体,A=奥氏体)

上述图1显示:热物理性质是相成分和温度的函数,可表示为

(),()k k k T T ψξξψ=∑ (5)

这里的ψ代表,,p c λρ或,k ξ是第k 相的体积分数,k 分别表示为A (奥氏体),M (马氏体),B (贝氏体),P (珠光体),F (共析铁素体)

注: 9

潜热

由相变产生的内热源的密度由(6)计算

k k d q H dt

ξ=? (6) 这里的k H ?是当奥氏体转变的时候,每单位奥氏体与每单位第k 相的焓差。如表-1所示。k d ξ是第k 相在dt 中的体积分数。

热传递系数

当在空气中冷却时,综合的热传递系数包括:辐射传递系数()r h 和对流转移系数()c h ,即

air r c h h h =+ (7)

辐射传递系数()r h ,表示为:

()()22r s a s a h T T T T εσ=++ (8)

这里ε是表面热辐射系数,本文中取为0.6,σ是Stefan-Boitzmann 常数,σ=5.768)/(10428k m w -?

在空气中冷却的对流转移系数的经验公式可近似地表述为

c h = (9)

油和搅动的水的热传递曲线是反向热传递方法取得(IHTM)。首先,从数据采集系统取得冷却曲线;这个数据采集系统在不同位置有平探头,尺寸120mm ?120mm ?20mm ,如图2所示。然后,这些冷却曲线输入基于ZHTM 的有限差系统。从而计算出热传递系数。图3所示是计算之中和搅动的水的热传递系数。用的槽车,尺寸为4.5m ?7.5m ?4m 。

注:

10

2.5相变计算

在热处理中的相变通常分为扩散型相变和元扩散型相变(马氏体转变) 对扩散型相变,持续的冷却曲线可能被打断,在极短的时间间隔的恒温区。孕育期采用的是Scheil 可加性法则来确定,也就是标志相转变开始时间John-Mchl-Avraml 公式用来确定潜伏时间和评定微观组织的体积分数

Scheil 可加性法则:

1()i i

t T τ?=∑ (10)

John-Mehl_Avarami 反应式:

注: 11

1exp()n f bt =-- (11)

潜伏时间:

()11*ln 1i

n i i i f t b --??=-????

(12) 潜伏期体积分数: **1exp(())n i i i f b t t =--+? (13)

真正的体积分数:

*11max ()i ri i f f f f f --=+ (14)

这里)(T i τ表示在一定T 下的潜伏期,i t ?表示在一定温度下短暂的等温时间。b 和n 是两个常数,它们由TTT 图上是新相体积分数从1%和99%的两条不同曲线得到。1-ri f 和i f 是新相在 i-1和i 中的体积分数,1-ri f 是奥氏体在i 时间的体积分数max f 新相转变开始的残余奥氏体的体积分数

对马氏体转变,马氏体的数量仅仅是温度的函数,其体积分数由Koistinen-Mar.burger 法则得出:

1exp(())s f M T α=-- (15)

这里s M 是马氏体转变开始温度。α是一个常数,对于AISI.P20钢。0.023α=

3. 结果与讨论

商用FE 软件,MSC.Marc 和用户定义的子程序用于分析AISI.P20钢热处理过程,大尺寸的P20钢1700mm ?1000mm ?460mm.由于对称性,由一个高度的几何条件需要考虑,通过试探法来确定网孔的尺寸,图5所示用膨胀计测试的AISI.20钢的TTT 图。用膨胀计为了便于热处理过程中相转变的计算。

注:

12

3.1油淬

油通常作为AISI.P20钢的淬火介质,来避免在热处理过程中的淬裂额,图3所示油的热传递系数,在860C 奥氏体后。然后大尺寸的钢被油淬,图6所示为油淬后沿中心轴的计算的微观组织分布状态

如图6示,没有发现马氏体,贝氏体也很薄,从钢表面大约10mm ,深处珠光体开始出现。当铁素体体积分数达到峰值12.5%时,从表面大约50mm 深至中心的主要相是珠光体,因此,对大尺寸的P20钢,油淬是不合适的。

3.2 直接水淬

因为油淬的淬硬层太薄,所以搅动的水淬来代替油,在860C ,奥氏体化后,钢被水淬至室温,图7所示是大尺寸的模具钢淬火后沿中心线计算的微观组织分布状态。

从图3(b)中可以看出搅动的水具有很高的冷却能力,当铁素体出现在钢表面100mm深时,珠光体出现在表面下30mm深处。在中心铁素体体积分数增加不足10%,所以,对于想获得更合适的微观组织的大尺寸钢,水淬是合适的虽然,通过直接水淬,能够获得较好的组织分布。但是,在实际热处理中,淬火裂纹总是发生。裂纹通常发生在钢的角落。所以,对于更合适的热处理过程,需要进一步的研究。

3.3 水淬+空气预冷

从图8中可以看出,在直接水淬过程中的裂纹和角落周围的马氏体转变相关。为了克服这个问题,本文中,将设计和模拟新的热处理过程—水淬+空气预冷。

图9所示是空气预冷1200S后的珠光体分布状态。从图9(a)中可以看出,预前空冷合适的时间,角落的奥氏体转变为珠光体,最终降低了在后续的水淬过程中产生裂纹的倾向。图9 b,e,d所示在空气预冷过程中非常薄的珠光体层在角落周围,在钢的大部分区域,奥氏体没有分解,在后续的水淬过程中将持续分解,和直接水淬一样。

注:13

注: 14

在大尺寸P20钢的实际热处理过程中,证明,空气预冷能有效地避免淬硬裂纹,但是,一些小的裂纹仍然产生。这些裂纹位于表面附近。它们是在水淬过程中马氏体转变时产生的。为了使大尺寸AISIP20钢的热处理过程最佳化。本文将设计和模拟另一个热处理过程:空气预冷+水淬+自我回火。

3.4 空气预冷的水淬

图11所示是沿中心线的不同位置的冷却曲线。热处理过程包括4个阶段:预冷1118s ;第一次水淬3892s ;第一次空冷201s ;第二次水淬2380s

图11所示为模拟结果。第一次水淬后,表面温度接近100C ?,低于s M 点,所以,马氏体开始转变。如果继续在水中冷却,钢很容易产生淬火裂纹。此时,钢从水中驱除进入第一次空冷,从中心的热传递将增加表面温度在200C ?以上。因此,转变的马氏体被回火,它的脆性降低,但是短暂的回火不会影响中心的冷却速率。第二次水淬后表面温度低于100c ?中心温度降低接近300C ?。所以,钢被取出后,放在空气里,新的转变的马氏体和表面附近的贝氏体,类似于自我回火,这种方法有效于避免淬硬裂纹。用这个方法形成的组织状态和直接水淬的形成组织状态基本相同。

结论:

根据热传递和微观组织状态的数学模型,大尺寸的塑料模具钢的热处理过程进一步研究。阐述了水能够取代油,作为淬火介质。空气预冷的水淬加自我回火是理想淬火过程之一。

17001000460m m m m m m ??AISI P20钢奥氏体化后第一次空冷直至角落

周围只有薄薄的一层珠光体,而这些角落是最容易产生淬火裂纹的地方。在后续的水淬过程中当表面转变为马氏体时,钢从水中转至空气中,即新形式的马氏体自我回火。模拟结果清楚的显示了预冷和自我回火不会降低淬硬深度。其微观组织状态与直接水淬得到的组织相似,但是后者更能有效阻止淬火裂纹,比油淬能获得更深的淬硬层。

致谢:

此设计项目是由中国科技部支持(NO.MT1:17)作者诚挚的感谢曾经帮助过

的老师和同学,

注:15

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