[Cedric Eveleigh]

If 4130 steel in heat affected zones is harder and more brittle because of rapid cooling, would avoiding the use of heat sinks reduce the amount of hardening and embrittlement in the heat affected zone because of slower cooling?

I thought of this question after reading this little welding tips and tricks article: http://www.weldingtipsandtricks.com/welding-metallurgy.html

 

[customfab]

I've always been a little skeptical with using heat sinks in frames, apart from the seat cluster. Reason being is that every joint is a thick to thin joint anyway and the open place where putting a heat sink is an option, is the thicker of the two tubes anyway. Putting a heat sink in the thinner tube would be great, but that's always the tube and can't easily be done.

To get a real answer to this you're going to have to weld some coupons and send them off to a lab, or at the very least do some hardness testing.

 

[Nova_Guy]

Heat sinks and hardening-

Actually 4130 with TIG welding in particular creates a hysteresis curve-

sort of like a goofy sin wave with the highest value (amplitude) just behind the toe of the weld. This is well documented for many steels.

So you end up with 4 zones- stay with me on this and try to visualize from right to left in the diagram:

- Harder than delivered zone (usually about 4-7mm) from the weld toe centerpoint outward
- Crosses the zero axis (the original delivered strength)
- Softer than delivered zone- after crossing the zero, 4130 goes to a lower value that is dependent on how much time and heat were applied, followed by how quick did the tube cool.
-Return to delivered (before welded value) the bottom of the curve usually returns at a slower rate than the high value (more distance to get beck to the delivered value.

So why is this important? a heat sink will slow down the cooling process so that you have less of a contraction or distortion problem. This really makes a difference in AL7005 where the expansion/contraction is 3x's what steel is.

If someone is fast and uses the minimal heat, they end up with less change in the strength values of the tubes and less distortion. If you use more heat and are slower, well, you get the other result.:skep:

 

[Cedric Eveleigh]

[...] a heat sink will slow down the cooling process so that you have less of a contraction or distortion problem.

How would a heat sink slow down the cooling process? Wouldn't it accelerate the cooling process?

However, I understand that heat sinks can prevent overheating and therefore that they can be beneficial with regard to material properties.

 

[Nova_Guy]

How would a heat sink slow down the cooling process? Wouldn't it accelerate the cooling process?

However, I understand that heat sinks can prevent overheating and therefore that they can be beneficial with regard to material properties.

Simple- a heat sink is a larger body with larger mass-the mass takes more time to heat and cool. There's more to say about the thermal conductivity of specific materials, but the mass is the most significant

 

[Cedric Eveleigh]

Hmm. Ok, so could it be that heat sinks slow the cooling process if they've heated up enough (after lots of welding), but accelerate the cooling process if they haven't heated up much (after initial welding)?

 

[Nova_Guy]

not sure on your idea. Steel is a fairly good conductor of heat-the heatsink is directly in contact- wicking and conducting heat.

One thing you might be forgetting is that the inside of a thinwall tube has a similar surface area to the outside diameter.
For instance- the circumference of a 28.6 0.8/0.6 mm wall seat tube inside and outside measurement are very close to the same value (89.85mm vs 86.08mm). When in contact with a heat sink, the ability of inside diameter (roughly 49% of the dissipative area- circ x length) to cool in "air" significantly reduced. So both mass and surface cooling are in play retarding the cooling cycle.

We used to make and sell bell brass heatsink plugs for ST, BB, etc back many years ago when builders were really struggling with the first gen. super thin hardened materials like Prestige UL. The heat sinks were a great help in fighting ovalization and creep.