Heat Treating of Super Alloys — Understanding Industry Terms

Understanding the terminology used on various (Blue) Prints and Material Certifications requires a solid mechanical engineering background or years of experience. The main reason is how much the terminology varies depending on manufacture, distributor and (Blue) Print specifications.

What we’ll cover

This article highlights the variations in terminology that are currently being used in the industry. Hopefully, it will provide insights for both engineering management and non-engineering support staff. Included are descriptions of  the terminology used for many Super Alloys— which are principally Nickel, Chrome, Cobalt, and Iron alloys of stainless steels.  Some of these alloys include industry trade names such as Inconel™, Hastelloy™, Haynes™, Monel™, Nitronic™,  Waspaloy™, and many others.

The specifications

These materials are typically specified and purchased per an American Military Standard designator, such as AMSXXXX,  which describes the alloy, materials shape, method of manufacture and testing, and heat treat condition.

Long form and Short form certifications

Typically, a “Short Form” certification would have the alloy chemistry and method of manufacture reference testing specifications to an industry standard, such as ASTM, AMS, ASME, SAE, etc.

A “Long Form” certification will have all information that is in the “Short Form” certification with additional information regarding the “physicals”. The “physicals” portion of the certification provides the actual strength of that specific batch (called “Heat”). This information is provided in Tensile Strength, typical in units called KSI (1000 PSI). Long Forms may also provide non-destructive testing (X-Ray or Ultrasonic), grain size information, as well as hardness or other information required to meet an Industry test specification.

Be careful when looking at material certifications. Typically, AMSXXXX-designation materials are not heat treated to a hardened condition. But a “Long Form” certification may state the material is “Capable” or “Sample was tested to a harden condition.”  Tensile values may be given for that “sample test”, but this was actually a  “capability test.” This does not mean the material provided is in the hardened condition.

The three most common conditions of stainless steel

When Stainless Steels are purchased, they typically are acquired in one of three conditions: Annealed,  Solution Heat-Treated, or Precipitation Heat-Treated. Depending on the alloy, the most common conditions are Annealed or Solution Heat-Treated.

1.) Annealed

Annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above the recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling1. This is common condition for Chromium/Iron  based stainless steels. The alloys like 17-4PH, 13-8Mo, 15-5PH.  This is typically the softest and most ductile condition, great for forming, but maybe too gummy for machining.

2.) Solution Heat-Treated (also called “solution hardened”)  

Solution heat-treated is a preliminary heat treatment which softens and uniformly distributes the natural  internal  stresses in the material creating a more homogenous material.

In this heat treatment process, the  alloy is heated to a high temperature, single-phase zone to maintain the temperature. The excess phase is sufficiently dissolved in the solid solution and then rapidly cooled to obtain a supersaturated solid solution.

Because the operation process is similar to quenching, it is also called “solution hardening.” It is suitable for alloys in which the solid solution is used as a matrix and the solubility changes greatly when the temperature changes. It is generally a preliminary heat treatment, and its role is to prepare optimum conditions for subsequent heat treatment (1).

Why is it performed? The manufacturing processes that form the substrate undergoes plastic deformation where the structural material experiences loading and unloading in the plastic range. When this occurs, the structural  material has undergone a great change, which is called the hardening phenomenon.  This forming process may increase localized deformed areas yield strength.

The Solution Heat-Treatment can eliminate the localized residual stress,  creating a uniform yield strength through the structure.

The purpose is to make the alloy more uniform and eliminate  residual stress generated from forming process, or welding processes. The resulting process is a relatively soft and ductile condition. The term “Solution Hardening” is somewhat misleading because the material is soft after performing the Solution Heat-Treat process.

This is commonly produced from the mill condition for Nickel and Cobalt/Iron-based Super Alloy Stainless Steels. The alloys like Inconel™ 625, Inconel™ 718,  A286, Haynes™,  etc. is a Solution Heat Treated or Annealed condition.

3.)  Precipitation Heat-Treated (also called precipitation hardening, aging, age hardening, particle hardening, STA)

Precipitation hardening, also called age hardening or particle hardening, is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminum, magnesiumnickel, and titanium; and some steelsstainless steels, and duplex stainless steel. In superalloys, it is known to cause yield strength anomaly providing excellent high-temperature strength.

Precipitation hardening relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal‘s lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden the material.

Precipitation in solids can produce many different sizes of particles, which have radically different properties. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called “aging.” Solution treatment and aging is sometimes abbreviated “STA” in specifications and certificates for metals.

Two different heat treatments involving precipitates can alter the strength of a material: solution heat treating and precipitation heat treating. Solid solution strengthening involves formation of a single-phase solid solution via quenching. Precipitation heat treating involves the addition of impurity particles to increase a material’s strength.(2)

This process creates increased yield strength in many malleable materials and creates a desirable  strength and hardness level for the final application.

Protomatic can help

As you can see, the terminology used by industry for describing the heat treating of super alloys is varied and complex. We hope this article serves as a useful tool and guide, and answers some of your questions. If you have questions or would like to know more about heat treatments or anything else to do with precision CNC machining, contact us at Protomatic using the addresses below.

Scott Allen, Sales Manager        Scotta@protomatic.com
Lori Frantz, Sales                      Lori@protomatic.com
Doug Wetzel, VP/GM                Doug@protomatic.com
Brian Heldt, Program Manager   Bheldt@protomatic.com

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About the author: Doug Wetzel is Vice President and General Manager of Protomatic. Protomatic is a CNC precision machining shop specializing in prototype and short-run production components for the medical, aerospace, and other technical industries. Because of the critical nature of the parts they design and manufacture, the emphasis is always on Life-Saving Precision.

(1)   Wikipedia References/Definitions, Solid solution Strengthening
(2)   Wikipedia References/Definitions, Precipitation Hardening