When machine shops fabricate or maintain heavy-duty components, they will eventually encounter a request for a precipitation-hardening stainless steel. The 15-5PH grade is one of the more popular choices, as it provides a decent balance of strength, corrosion resistance and toughness. Understanding the characteristics of the material on the shop floor is essential to successful design of structural, lifting, and rotating equipment.
The 15-5PH grade is typically purchased to the ams 5659 specification. It covers bars, forgings and rings, and specifies the chemistry, mechanical properties and the condition of the material. If a specification is not included on a purchase order, suppliers may interpret the customer’s request for “15-5PH” in a variety of ways, causing quality concerns. Referencing ams 5659 on the purchase order gives the customer’s quality department a clear standard when investigating the condition of the material on receipt.
Heat Treatment Condition A
The most important condition of the material is referred to as the heat treatment condition. In the case of 15-5PH, this refers to the condition before ageing. The material is relatively soft prior to ageing and therefore easier to machine. After machining, the components are aged at the desired temperature to build up the required strength. The ageing temperature also affects the toughness and the resistance to stress corrosion cracking of the material. Lower ageing temperatures result in higher strengths, while higher ageing temperatures result in better toughness and stress corrosion cracking resistance. Designations H900, H1025 and H1150 refer to the ageing temperature of the components, with lower temperatures giving higher strengths.
A customer has two major advantages to machining in Condition A and subsequent ageing. The first is better tool life. The second is shorter cycle times. It is worth considering a minor, predictable dimension change during ageing. Certain features may be left with an allowance during machining. After ageing, these features may be finished to size. Examples of these features are bores and threads.
Machining and Fabrication Practicalities
Compared to other austenitic stainless steels, 15-5PH is easier to machine. However, several considerations still apply. Working harder to machine 15-5PH may actually work harden the tooling and cause poor surface finish. Worn tooling may cause an uneven surface finish and may quickly ruin the tool. With long production runs, planners should anticipate tool changes.
Where fabrication is required, planning the weld joints is critical. 15-5PH is technically weldable, however the properties of the weld zone and heat affected zone will not match the aged properties of the base material. Consequently, many engineering firms avoid welding 15-5PH on critical load bearing areas and instead machine from a forged or bar stock blank. If welding is required, agree upon the procedure, filler material, and post weld heat treatment with the customer, and your supplier, before metal is cut.
Checking supplier credentials and certification
For a safety-related component, the paperwork is part of the product. When sourcing material for loaded or rotating assemblies, require your supplier to provide a mill test certificate for the heat or batch you are receiving. Ensure the heat number on the certificate matches the marking on the bar, and that the specified condition and chemistry matches your order.
It is reasonable to expect your supplier to be able to provide traceability to the original manufacturer. It is also reasonable to expect your supplier to be able to perform independent testing and positive material identification (PMI) if required by your customer. Maintaining these records with your inspection reports facilitates the handling of customer audits, inspections and/or queries. This is especially useful in industries where equipment is inspected on a cyclical basis.
Choosing the right form and size
Closer to the finished size reduces waste and machining time and cost. Round bar is the usual choice for fittings, pins and shafts. Other shapes may be better served by a forged ring or blank. It is better to discuss lead times and cut-to-length services early, especially for a tight delivery.
When 15-5PH is the better choice
There are many considerations when choosing a material. This grade is often the better option when strength, corrosion resistance, and dimensional stability are important in the same application. This could be a fitting, a shaft, or a pin. In the case of a structure where these properties aren’t as important, lower-cost, more conventional steels could be considered. Depending on the scope of the project, a quick chat between a design engineer, procurement and the machine shop when a quote is requested may help make the right choice.
There are certain applications where demanding engineering work requires the use of 15-5PH. Among others, consideration should be given to the clear specification of the requirement, the condition of the supply, and the terms of the agreement. If your suppliers are involved in the planning process, your team is most likely to be on time and meet the performance requirements of your customers.