In this topic, I want to talk about the mechanical properties of stainless steel, especially the tensile properties and impact properties. In the former lecture, we already learned about the four main families of stainless steel which are ferritic, martensitic, austenitic, and duplex stainless steels. These four categories of stainless steel are divided by or based on differences in chemical composition and microstructure. And because of these microstructural differences or chemical composition differences, they have very different stress-strain curves in tensile tests. Firstly, ferritic stainless steel usually has low strength and low ductility as shown in this diagram. Here, this graph shows 11 percent chromium ferrite, and this one shows 17 percent chromium ferrite. They usually have very low yield strength around 250 MPa, and the tensile strength ranges from 300 to 450 MPa. And their ductility is around 30 to 40 percent. The second one is martensitic stainless steel. Martensitic stainless steel usually has very high strength up to around 1,000 MPa and sometimes over 1,000 MPa. However, their ductility is very limited, usually less than ten percent. And the third one which is the most popular stainless steel is austenitic stainless steel. They have very low yield strength. Low yield strength. However, they have very good capability of high strain hardening. So as the deformation goes on, their strength increases uniformly and also monotonously. And you can see here, they have very high UTS and also very high ductility, up to around 50 or 60 percent. So their large elongation gives very good formability of austenitic stainless steel. Finally, duplex stainless steel usually has properties somewhere between ferritic and austenitic stainless steels. So they have a medium range of high strength and also some limited ductility. However, since they have very good properties in corrosion, they are used in specific applications. When you want to understand the mechanical properties or mechanical behavior of austenitic stainless steel, the stacking fault energy is a very important factor because it determines the deformation mode. This stacking fault is usually formed by the dislocation dissociation which is usually formed in austenitic stainless steel. In order to make this kind of stacking fault, we need some energy which is called the stacking fault energy. And depending on this stacking fault energy, the deformation mode changes. That means if the stacking fault energy is very low, alpha prime martensite which is shown in this figure or epsilon martensite in this figure is formed during deformation. And if the stacking fault is higher, then we can see this kind of mechanical twin during deformation. However, if the stacking fault is very high, we cannot see this kind of mechanical twin or martensite during deformation and we can only see dislocations. Many alloying elements sometimes increase the stacking fault energy and sometimes decrease the stacking fault energy. Usually nickel increases the stacking fault energy very effectively. However, chromium and manganese decreases the stacking fault energy. And carbon increases the stacking fault energy. However, nitrogen sometimes increases or sometimes decreases the stacking fault energy depending on base chemical composition. So we need to study a little more about this stacking fault phenomenon and also the effect of alloying elements on the stacking fault energy. Depending on this stacking fault energy or stacking fault formation, the stability of austenite is determined. That means if austenite is unstable during deformation, we can see the formation of alpha prime martensite or mechanical twinning. And if the stability of austenite is very high – that means if the austenite stability is very good – in that case, we don't see any more alpha prime formation or epsilon martensite formation. So, austenite stability is determined by stacking faults, and these stacking faults increase with temperature. That means if the temperature is low, stacking faults occur more easily, and because of that the deformation also changes with temperature. Alpha prime martensite or epsilon martensite, which was shown in the former slide, is called a strain-induced martensitic transformation. So this strain-induced martensitic transformation changes the shape of stress-strain curves as shown in the left figure. As you can see here if the temperature is a little bit high, then you can see that stress-strain curve increases just monotonously. However if the temperature is low, for example at minus 25 degrees Celsius or minus 80 degrees Celsius, you can see the change of the curves here, and there is some inflection point. That means at this point alpha prime martensite is forming. And sometimes we express this austenite stability using this number, Md30, which means that after 30 percent of cold rolling about 50 percent of austenite is transformed to martensite, then we call that temperature Md30 temperature. So if the Md30 temperature is higher, the austenite is more unstable. Here you can see the difference between AISI 301 austenitic stainless steel and AISI 304 austenitic stainless steel. And the Md30 temperature is a little bit higher than room temperature, and here the Md30 temperature is lower than room temperature. The y-axis shows the alpha prime martensite content during the deformation. As you perform cold rolling, the alpha prime martensite is increasing like this one. And if the Md30 temperature is higher, the formation of alpha prime increases. And as you can see here the stacking fault energy for this alloy is 4.4 millijoules per square meter. However, in this case, the 304 case, the stacking fault energy is around 17.7. It is much higher than this 301 stainless steel. That’s why it shows a more stable phenomenon. Another important mechanical property of stainless steel is toughness, which is usually determined by an impact test. In this diagram, you can see the differences between BCC metals and FCC metals. Here in BCC metals, usually they have a very pronounced temperature dependence and also a pronounced strain rate dependence. That means if the temperature decreases, then the change of yield stress is very drastic. However, in austenitic stainless steel, FCC metals, usually the dependence of this yield stress on the temperature is very weak. The change is very slow although the yield stress increases as the temperature decreases. However, the degree of increase is much lower than in BCC metals. One more thing we have to know is the fracture/cleavage strength. In this case, usually this strength is independent on temperature. So if this fracture strength is constant and if you decrease the temperature, the yield strength increases. And at this point the yield strength is larger than the fracture strength. That means when competition occurs between yield strengths and fracture strength, since the yield strings is larger than the fracture strength, before yielding, fracture occurs. However in FCC metals, it is always lower than the fracture strength. That means fracture or brittle fracture does not occur even though the temperature is decreasing. So at this point, the transition between yielding or ductile fracture to brittle fracture occurs, and this is called the brittle ductile transition temperature. So in order to avoid or in order to inhibit this kind of brittle fracture, we need to refine the grain size – that means to make the grain size smaller and smaller. And if the fracture strength is much larger, then we can reduce brittle fracture. Or even though we increase the yield strength, if the amount of increase is smaller, then we can reduce the tendency of brittle fracture. And one more thing very important is eliminating grain boundary contamination. Impurities like P and S which are very very harmful elements which contaminates grain boundary. So by the contamination of these elements, this fracture strength decreases very rapidly. And then you can see the increase of DBTT from this point to this point occurs. That means if DBTT or BDTT is increasing, it is much more harmful in fracture toughness. Here you can see the differences between alloy categories, and in this diagram we have austenitic family, duplex family, ferritic family, and martensitic family. Here you can see that only the austenitic grade does not show a drastic change of fracture mode from ductile to brittle. However, for the ferritic, martensitic or duplex phase, since they all have some portion or 100 percent of BCC crystal structure, at some temperatures or at some point they show a drastic change of fracture toughness. So this one shows the differences among the alloy categories. And austenitic stainless steel has the best toughness. Even at very very low temperatures – at the liquid nitrogen temperatures – they maintain a very high value of impact toughness. That's why they are used in the very low temperature atmosphere, cryogenic uses. However, there is one exception in austenitic stainless steel which contains a very high amount of nitrogen, and in that case it shows exceptionally some DBTT phenomena. Usually, ferritic stainless steel, martensitic grade, and martensitic stainless steel all show ductile-to-brittle transitions, and ferritic grades or martensitic grades have a very high DBTT temperature. So this is very dangerous. Chromium and carbon increases DBTT, so we need to reduce chromium content and carbon content in order to reduce DBTT. And also, as I talked in the former slide, the fine grain size is required to obtain a very good property of impact toughness. Finally, I want to talk about some future development directions for austenitic and duplex stainless steels. There are two main directions. The first one is low cost, and the second one is the resistance to extreme corrosion properties. Even though the price goes up high, however, if we need a very high resistant property in order to cope with extreme corrosion properties, then we need to develop a new kind of stainless steel. So these two categories – low-cost and resistant to extreme corrosion properties – are the main directions of new stainless steel development. And one more thing I want to add is high strength without losing ductility for lightweighting, especially in the automotive industry. In this diagram, you can see the direction of duplex stainless steel and austenitic stainless steel. This is the PREN number which means the pitting resistance equivalent number, and this is raw material cost. We need to reduce the cost, and also we need to increase the PREN number. And this is the final statement I want to say in this stainless steel lecture.
