菜哥 发表于 2020-12-27 21:20
我请教一个问题，碳含量低于0.025%的钢，奥氏体向铁素体转换，并形成渗碳体的过程，是以下哪一种？
1.碳原 ...
@Architect，@x
我在想这个问题的时候，犯了一个错误。我假定讨论的是碳含量低于0.025%的钢，但却总想着“奥氏体中的碳含量高于铁素体”，所以就觉得，奥氏体向铁素体转变，一定会有碳析出。
问题就出在“奥氏体中的碳含量高于铁素体”这句话上，这个不准确，准确的说法应该是，“在奥氏体和铁素体两相同时存在的温度下，碳在奥氏体中的溶解度高于铁素体”，同时，也应该考虑到，降温过程中，上述两相的相对含量变化以及碳在上述两相中的溶解度变化。就是要把温度变化的影响考虑进去。
碳含量低于0.025%的钢，从奥氏体温度区间缓慢降温（即平衡状态）至室温的组织转变过程：在铁碳平衡相图中，在横坐标上选取碳含量低于0.025%的一点，作一条竖直向上的直线，从奥氏体温度区间向下，该直线与铁碳相图的图形有三个交点。在平衡降温过程中，温度先降至最上一交点所指示的温度，这就是奥氏体向铁素体转变开始的温度。温度继续下降，到达第二个交点所指示的温度，所有奥氏体均转变为铁素体。在此过程中，虽然碳在铁素体中的溶解度比其在奥氏体中的溶解度低，但在降温过程中，碳在奥氏体和铁素体中的溶解度都是逐渐增加的，可以容纳新的铁素体形成后多余的未固溶于铁素体之中的碳原子，而且，随温度降低，转变持续进行，铁素体所占的比重逐渐增加，容纳碳原子的能力也在增加，直至所有奥氏体转变为铁素体。所以，奥氏体向铁素体的转变过程中有碳原子的扩散，但没有渗碳体的形成。
继续降温直至第三个交点（即最下方的交点），在此过程中只有温度变化，没有相变发生和渗碳体形成。从第三个交点开始继续降温至室温，碳在铁素体中的最大溶解度会随温度降低而减小，就是这个时候，才有碳原子以渗碳体的形态，在铁素体晶界析出。
说一下为什么没有珠光体形成？先明确一下，珠光体不是铁素体与渗碳体的混合物，应该是铁素体与渗碳体的层片状机械混合物，注意那个“层片状”，这个很关键。珠光体的形成条件：碳含量为0.8%的奥氏体，在缓慢降温过程中（平衡条件），达到723摄氏度，由一相转变为两相（低碳相铁素体与高碳相渗碳体）的层片状机械混合物。我把讨论的碳含量，限定在了低于0.025%的区间，从铁碳相图看，在降温过程中，没有得到碳含量为0.8%的奥氏体，所以不会有共析反应发生，也就不会得到室温下的珠光体。
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When I try to find a solution for this question, I make a mistake.I limitthe discussion on the steel with a carbon content less than 0.025%, but I keepa thought in my mind, “carbon content in austenite is always higher than thatin ferrite”. So, I insist on there must be a specific amount of carbon will separatesout in the form of cementite during the transformation of austenite to ferrite.
I amwrong with “carbon content in austenite is always higher than that in ferrite”,it’s inaccurate. A more accurate expression should be “within the temperaturerange that both austenite and ferrite exist, the carbon solubility in austeniteis higher than that in ferrite”. Meanwhile, we should take into account theeffects of variation in temperature, i.e. variation of amount of two phases andcarbon solubility in them caused by variation of temperature.
Phase transformation in steels with carbon content less than 0.025%, on slow cooling under equilibrium condition, from austenitic temperature range to room temperature:Select apoint on the abscissa indicates a carbon content less than 0.025% iniron-cementite phase diagram. Draw a vertical line from the selected point, itintersects the diagram at three points from austenitic temperature range to room temperature.Thehighest intersection point is reached first on cooling, that’s the temperatureat which transformation from austenite to ferrite begins. As the coolingprogresses, the second intersection point is reached, at that moment, all theaustenite is transformed into ferrite. During this process, the carbonsolubility in ferrite is always lower than that in austenite. However, thesolubility of carbon in ferrite and austenite increase as the temperaturedrops. Thus, they can provide enough room for the excess carbon atoms that aresqueezed out from newly formed ferrite. Furthermore, the transformationprogresses as the temperature drops, more ferrite is available to accommodate carbonatoms. This process continues until the transformation is completed.So,during the transformation, diffusion of carbon atoms is involved, but there isno separation of carbon in the form of cementite.
Oncooling from second intersection point to the third one (the lowest one), nophase transformation or cementite formation takes place, just drop intemperature.Oncooling from lowest intersection point to room temperature, the solubility ofcarbon in ferrite decreases as the temperature drops, and excess carbon atoms separateout in the form of cementite, along the ferrite grain boundaries.
Why no pearlite can be obtained at room temperature?First ofall, we should make it clear that, pearlite is not a kind of mixture of ferriteand cementite, it is a mechanical mixture of alternate layers of ferrite andcementite. “Alternate layers” should be taken as a key term, for its animportant characteristic of pearlite.
Theconditions required for the formation of pearlite:Austeniteof a carbon content of 0.8%, on slow cooling (under equilibrium condition), reaches723 degree celcius, austenite decomposes into two phases (low-carbon ferriteand high-carbon cementite) that shown as mechanical mixture in alternate layers.As we are talking about steels with less than 0.025% carbon, according to iron-cementitediagram, we fail to obtain 0.8% carbon austenite, that is to say, no eutectoidreaction takes place, then, no pearlite can be obtained at room temperature.
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