How is steel tempering addressed when installing pre-heat rivets?

Question

AFAIK when you heat up a piece of steel to high temperature and let it cool down slowly (as opposed to shock cooling by say submerging it into water) steel gets softer - that's called tempering. Steel getting softer means it can bear less load without getting deformed.

Suppose we connect parts of steel skyscraper with steel rivets. We heat them up in fire and then put them into holes and deform their tails. While the rivet is inserted inside a rather cool surrounding construction (and being deformed) it cools down which more or less resembles tempering process.

So it looks like installing rivets leaves them tempered and so rather soft and prone to deformation.

How is this process of rivets getting softer and "weaker" accounted for?

Hopefully someone with more metallurgy experience will jump in, but as I understand, tempering only applies to steel that has first been hardened (by quenching.) Most structural steels are not quenched, and therefore are already in a relatively soft state. This is because one of steel's big advantages as a building material is its ductility, which goes down as hardness goes up. Some high strength bolts are quenched and tempered, and as such are susceptible to damage from high temperatures. — Ethan48, Mar 14 '15 at 14:31
Correct, tempering only applies to previously quenched, martensitic steels. Tempering is a heat treatment which relieves the internal stresses associated with the rapid martensitic phase transformation and associated volume change. Tempering allows recovery of considerable ductility with minimal strength loss in martensitic steels of done correctly. — do-the-thing-please, Feb 06 '16 at 00:39

score 5 · Accepted Answer · answered Mar 14 '15 at 19:48

ASTM A502 covers rivets for structural situations. The linked version of the standard is from 1976, but it is probably safe to assume that riveting technology hasn't changed much since then. The latest ASTM A502 standard is from 2009 though.

The rivets are made from what is typical structural steel to begin with. This means that weathering, "Grade 3" rivets are made from ASTM A588 steel. A588 is a common designation for structural steel with atmospheric corrosion resistance.

The actual rivet isn't heated to a high enough temperature to change the steel structure as this article on rivets states:

Rivets were heated to 950°F - 1050°F, handbooks say. This is not hot enough for an austenitic transition, so it must only have been for causing thermal expansion...

Putting all of this together means that rivets start out with the material properties of standard structural steel and aren't heated enough to change those properties.

To state explicitly, the A3 curve on the Fe-C diagram is above 727 C, and 1050 F is about 570 C. To form austenite requires moving above the A3 temperature, so rivet properties shouldn't change significantly barring some ferrite recrystallization. Cooling times are probably low enough that grain growth does not occur to any significant degree as long as the rivets weren't received in a highly cold worked state. Nice answer! — do-the-thing-please, Feb 06 '16 at 00:41

score 1 · Answer 2 · edited Feb 05 '16 at 22:32

Structural steel are not usually intended to be heat-treated.

Heat treatment involves heating the steel beyond a critical temperature (typically around 850 °C) and then cooling it rapidly to change its crystal structure. This results in a 'stressed' structure which tends to increase tensile strength and hardness at the expense of toughness and ductility. The tempering process involves reheating hardened steel to a very specific temperature (typically in the rang 180-300 °C) to partially reverse this process with the specific tempering temperature allowing fine control over the balance between hardness and toughness.

However if the steel has not been quench-hardened in the first place then heating and slow-cooling will not have any great effect on its mechanical properties.

To put this another way tempering and annealing reverse the effect of quench hardening but won't soften steel which hasn't been hardened.

Generally structural steels are designed to have good tensile strength and ductility in a non-heat-treated state.

Clearly there is little point in hardening rivets for either hot- or cold-forming so once they cool they just go back to the state they were in before they were heated.

How is steel tempering addressed when installing pre-heat rivets?

2 Answers2