How heat treating impacts material specification

One of the keys to successful manufacturing of metal parts is understanding how the journey from raw material to finished product is impacted by material and design.
Heat treating is a vital step in that journey, so coordination between manufacturers and heat treaters should begin long before parts head from the factory to the furnace.
 

Dialogue during design

Heat treaters are sources of valuable information for manufacturers during part design. With intimate knowledge of how materials respond to heat treatment, metallurgists often advise manufacturers as they develop material specifications. This advice needs to be fully vetted and performance tested to verify requirements will meet design criteria before a final decision is made. Coordination between manufacturer and heat treater ensures material and treatment costs are minimized while part performance is maximized.
It can be a difficult balance to strike, but here are some of the questions a metallurgist can help answer so that material, design and process come together to make quality parts:

• What are the characteristics of the operating environment? Is it clean? Dirty? Hot? Cold?
• What stresses will the part encounter? Where will those stresses occur?
• Which heat treatment process will deliver the material quality the part needs to perform?
• Will high- or low-alloy materials extend or shorten treatment time? Which are best suited for the part’s intended function?
• What is the most cost-effective combination of material and treatment?

The examples below illustrate how this coordination adds value for manufacturers:
 

Example #1 – Automotive fineblanking

Assume that a manufacturer is designing for high-volume automotive fineblanking. The parts are 5 mm thick and will be carburized to an effective case depth of between 0.80 and 1.05 mm. No additional core hardness is needed because, in this operation, wear is the only design feature. The part will not be subject to impact.
AISI 1020 low-carbon steel is a popular choice in this case because low manganese content —and low hardenability— make it great for fineblanking. But a consultation with a metallurgist would reveal that it’s not the best choice because:

• Low-hardenability steel requires heat treaters to reduce loading in integral batch furnaces and increase heat treatment time —to around eleven and a half hours in this case— making heat treatment more expensive.
• Something a bit more hardenable, like AISI 1018, could be a good candidate because it has about twice the manganese as the 1020. But load reduction is still necessary for the 1018, and treatment times would still reach a little more than nine hours.
• Popularity of boron steel (such as 10B21) is increasing for fineblanking because it’s only marginally more expensive than the materials mentioned above but has significantly higher hardenability. That means heat treaters would not reduce loads and could accomplish the case hardening in as little as six hours. That represents a significant cost reduction, more than making up for the added material cost.

This scenario illustrates that increased material hardenability translates to better heat treatment efficiency, driving down total cost to manufacturers.
 

Example #2 – Precision tooling

Assume a customer is designing a precision tooling piece that consists of multiple segments that fit together in a precise cylinder. In the past, the customer has used both AISI S7 and AISI A2 shock-resistant steels for similar parts.
In this example, the customer intends to weld pre-machined segments together prior to heat treatment, something they’ve never done before. After treatment, final machining and electrical discharge machining (EDM) of the weld joints will be executed.
The customer could consult a metallurgist and learn:

• Higher-carbon and higher-alloy steels are more difficult to weld without cracking.
• S7 steel contains 0.50% carbon, 3.25% chromium and 1.40% molybdenum.
• A2 steel contains 1.00% carbon, 5.00% chromium and 1.00% molybdenum.
• Clearly, A2 would be harder to weld and then fabricate. The S7 is the better option.

Coordinate with your heat treater

The cases above show that heat treatment significantly impacts material specification. By coordinating with heat treaters while developing specs, manufacturers will balance material, manufacturing and treatment costs to get the best overall value.