Tubing Dimensional Changes During Heat Treating
Dimensional Changes from Heat Treating
A vexing problem for many metals manufacturers, including stainless tubing producers, is dimensional changes resulting from heat treating after a product has been completely machined or fabricated. Because the fabrication work is complete and heat treatment is the final (or nearly final) operation before the product is finished, such unexpected changes can be problematic with limited re-work options. Often, asking the customer for deviation or remaking the part completely are often the best (and yet least desirable) options to resolve such problems.
What causes Dimensional Changes during Heat Treating?
What causes this issue to occur, and why does heat treatment happen at the end of some manufacturing processes in the first place? Fabricating tubing and other machined components is readily accomplished when the product is hard temper, meaning that it has a higher tensile strength---or more simply, rigidity---when compared to annealed tube or bar. This distinction is important because when fixturing tubes for fabrication whether using traditional subtractive machining like mill/lathe work or newer means like electrochemical, EDM or laser, there is much less concern about damaging/bending the parts in production when working with hard temper tubing. Fabricating tube while soft (annealed) can be done, but risk of part fallout is much higher, especially if the tube tolerances are tight and the wall is thin. Therefore, it’s common to complete the fab work and then proceed to heat treatment to transform the product to the desired temper based on customer requirements.
Back to the earlier question of what causes the dimensional changes: several things can throw a wrench into gears of an otherwise well-oiled tubing manufacturing operation. Stainless steel tube chemistry varies lot to lot, with an allowed range for each of the elements comprising an alloy. For example, 304 stainless normally contains 8-11% Nickel, 17-19% Chromium and other lesser elements (with the bulk of the alloy being comprised of Iron). Small differences in chemical composition from lot to lot can play a role in tubing dimensional changes when exposed to heat treatment---and it’s nearly impossible to tightly control the exact chemical makeup because the composition is set far upstream when the metal is melted in an enormous batch for myriad uses in different industries.
The second factor is more controllable, and that is the quality of the furnace and the way that the product is ramped up/down to temperature. Poorly constructed or managed furnaces can lead to temperature changes that are too rapid, too shallow, or allow for uneven heating of the tubing as it’s being processed. Such inconsistencies can result in the part experiencing dimensional changes from a .001” up to 1/64”---and depending on tolerances, these changes can take the part out of tolerance. Changes can occur to any measurable feature on the tube, including outside diameter, inside diameter, length and hole diameters/feature locations.
An important consideration is whether the manufacturer heat treats in-house or uses a subcontractor. It is not uncommon for manufacturers to use subcontractors for heat treatment because of the relatively specialized nature of this equipment and the overhead costs of operating it. Vita maintains in-house heat treatment for most of the products it heat treats, using subcontractors for certain select operations that can’t be accommodated in house. These vendors undergo a qualification process and oversight via our quality management system.
If you require stainless tubing fabricated parts requiring an annealed or heat treated finish, providing generous tolerances where possible can provide some protection against the problems described above. At the very least, engaging in a conversation with the manufacturer regarding what controls they intend to use to prevent dimensional changes is appropriate. In some cases, the decision to perform fabrication operations while the tube is in an annealed state may be made despite the risks to avoid the alternative risk of the part going out of tolerance during heat treatment.