Machine Your Parts After Hardening, Not Before
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We published a three-part series on American Machinist as a guide to our process.
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Heat-treating is an important step for some parts, giving them higher reliability and performance as the material’s mechanical properties change under heat.
Through hardening, for example, can increase the hardness and strength of the metal. 17-4 grade stainless steel is commonly hardened from annealed to H900 condition which raises its tensile strength from 1100 MPa to 1380 MPa while also increasing hardness from 35 Rockwell C to 45 Rockwell C.1 This hardness improvement makes 17-4 one of the most commonly used materials where corrosion resistance is required such as oil and gas, marine applications, and surgical tools.
In conventional machining methods, metals are often first machined to rough shape while the material is in the softer, annealed state in order to reduce tool wear and increase machining speed. After rough machining, the parts are sent out for hardening prior to a finish machining or grinding operation to achieve final tolerance.
This multi-step process has a few challenges including:
Hardening causes distortions in the metal, which must be corrected with additional machining.
As parts are machined after hardening, internal stresses can be relieved leading to distortions which require extra stock to accommodate and negating some of the benefits to roughing in the annealed condition.
For very precise applications, re-fixturing the part in the machining cell after hardening can be challenging as the datum structures have moved or been distorted.
There are conventional machining solutions for addressing all of the above problems such as advanced cutting tool materials and tooling methods and high performance machining centers but they all add additional time or cost to the process.
Finally, there are some forming processes that require the material to be in the annealed state. Coining is a prime example where the impact forces during the operation could easily damage or wear the small features on a coining die if the workpiece material were hardened. In the example process flow diagram shown below, coining is used to create the staple forming pockets for a linear stapler anvil in a surgical tool. This process requires many extra steps just to accommodate the softened material.
Alternatively, if you could create these features directly in a hardened part using pulsed electrochemical machining (PECM), you could avoid multiple extra steps and the lead time required for outsourcing the heat treatment, thereby reducing time and cost while increasing part quality or consistency.
More generally, pulsed electrochemical machining (PECM) may offer a few benefits for through hardened parts vs. conventional machining operations.
Induced or relieved stresses from heat treatment are inconsequential when using PECM.
PECM can machine the parts in a single set-up or operation, maintaining the original datum structure throughout the process.
PECM can machined hardened or anneal materials at the same rate.
It is important to note that hardening the blank material prior to PECM is primarily relevant to through hardening, not case hardening. Although PECM can be used on case-hardened material, the part must first be processed to near-net shape prior to case hardening to ensure the hardened layer is not removed during final machining – whether that be PECM, CNC machining, or grinding.
If you are interested in saving time and money on your hardened parts, not to mention getting more reliable results than conventional operations, PECM may be a good fit for you. Contact us and share your part specifications. We’re here to help.