Stainless Steel

What makes stainless steel stainless?

Here you go!
From the link, you will see that there isn’t a single “stainless steel” – my copy of “Machinery’s Handbook” has pages of steel recipes in it.
The cool thing about Austenitic (300 grades) versus Martensitic (400 grades) is that you can tell them apart using a magnet. A magnet will only stick to Martensitic stainless steel. Most structural stainless steel (think lunch room equipment) is of the Austenitic grades. My stainless steel knife is Martensitic.
Another interesting stainless fact: it can rust. In the service, I remember storing HCl in a stainless-steel cabinet, and the fumes from the acid were sufficient to cause red rust to appear on the previously-stainless cabinet.

Stainless steel is an alloy containing chromium.

It takes about 13 percent chromium to make a steel a stainless steel. The chromium forms a passivating layer on the steel which inhibits corrosion. It also changes the phase composition of the alloy. Most steels (depending on their thermal history) are composed of ferrite (iron, with a small amount of carbon in an interstitial solid solution) and cementite (an intermetallic compound Fe(sub3)C). Stainless steels have a different iron phase present-- retained austenite. This is a face centered cubic form of iron, rather than the body centered cubic form.

Why is stainless steel so hard to cut and/or drill?

Thanks guys!

I saw a stainless steel sink that had been sitting outside for at least a decade, when my housemate and I realized that in all of our knowledge, we had no idea why it was stainless.

Sweet link minor7flat5.

I’m a pocket knife collector. It was explained to me that “stainless” means it stains less, not that it will never pick up any stain. Thus, a stainless knife, kept in the pocket and rarely cleaned will show a little rust. A stainless cabinet in an acid vapor area (or cleaned with acid and not rinsed thoroughly) will turn brown.

–Nott, whose fingers sometimes smell of Simichrome.

Philster, my Materials class is a little rusty, but I’ll try to help. First, check the information at this link.
It describes the crystal structure of the various steels and how to make them. It also includes the nifty iron-carbon phase diagram.

A glossary of terms can be found here.

The essence of the answer is the difference in the crystal lattice structure. You will notice technical terms like “body-centered cubic” and “face-centered cubic”. These are descriptions of the crystal lattice. Take iron atoms and space them into a cube, one at each four corners. This is the starting point. Now stick a ninth atom in the middle of the cube. This is the body-centered cubic lattice. See, there’s a central atom in the body of the cube. This structure has certain atomic strength based on the chemical bonds between the iron atoms. Alternately, take your cube and put a fifth atom at the center of each face of the cube. This is the “face-centered cubic” shape. The space between the iron atoms is called the interstitial spaces. The carbon atoms are smaller, and will squeeze into these interstitial spaces. In the FCC structure, they fit nicely. The more carbon atoms, the stronger the structure, because more atoms are bonded. However, the price is in ductility, or the ability of the metal to stretch under stress.

Strength is a measure of how much stress the metal can take before it breaks. (Stress is force per unit area, which is the correct quantity for measuring strength.) The stronger the metal, the higher load it can take before it breaks. However, metals have interesting stress-strain curves. That’s a plot of the amount of stress applied vs. the amount of deformation that occurs. Metal stretches. At the beginning of the curve, it is a straight line. This is called the elastic region of the curve, because any deformation (stretching) that occurs in this region will immediately return to normal when the stress is removed. Thus, like an elastic band, it will stretch and relieve without wear and tear. However, once you reach a certain value (called the yield strength), the curve takes a detour and shoots off to the right. This usually has something of a plateau, with varying shapes depending on the metal. It can be a long flat region with a slight upward slant, or a curvy up and down slope. This is called the plastic region. Any deformation in this region is permanent, and will not relieve when the stress is removed.

Consider a paperclip. If you bend the paperclip in half, and then straighten it again, you will notice the place at the corner is permanently crimped. That’s plastic deformation. At some stress level, the metal will eventually break. That’s called the ultimate tensile strength. Maximum stress it can take before it breaks.

The stress-strain curve looks something like this.

 |                .  ^
 |           .  ' 
 |       . '
 |      /
 |     /
 |    /
 |   /
 |  /
 | /

With Martensite, quenching (dumping it in water for rapid cooling) will lock the carbon atoms inside the iron atom lattice in the BCC state, distorting the shape. Thus a high strength, but at the cost of toughness (or ductility; they are mathematical inverses of each other). Toughness is the shape of the plastic deformation region of the above curve. If the line is long to the right, that means it can absorb a lot of deformation before breaking. Whereas if it is short, then once it reaches the yield stress, it very quickly is enough stress to break.

Tempering is reheating the steel to let some of the carbon diffuse from the crystal (mostly closer to the edges) and thereby relieve the distortion of the crystal lattice. This returns toughness, but slightly reduces the strength.

Another discussion of Martensites, and visuals of the crystal structures, including pictures of the microscopic structure.

Okay, I hope I’ve answered the question.

One more confusing point. Above I almost said there is more space between the atoms in the FCC than in the BCC structure. According to packing ratios, the BCC structure is actually less tight. But for some reason the carbon fits into the FCC structure and not the BCC structure. The sites I’ve linked confirm this, but I don’t recall why that is right now. I think it has to do with the layout of the spaces. Although the packing factor is higher (meaning less overall space), the layout of the space makes room for the carbon atoms, whereas in the BCC lattice, the spaces are more diagonal, so while there is more room, it is shaped funny. Sort of like the difference between a square room and a rectangular one. You can have a square room 10 ft by 10 ft (100 sq ft), or you can have a rectangular room 100 ft by 1 ft (100 sq ft). One is easy to move around in, the other is a very tight fit. Same thing for the lattice structures. That makes sense. That’s why the carbons fit in the FCC structure but distort the BCC in Martensite.

Now that deserves a big thank you!