HRSA is the most challenging material group in machining, and also the most expensive metals to purchase. Although not as popular in machine shops as steel or aluminum, due to the high material and manufacturing cost, it’s an important niche to master. The machinability ranges from 5% to 40%.

Bar Chart - Machinability of superalloys vs other metals
Bar Chart - Machinability of superalloys

What is a heat-resistant superalloy (HRSA)?

Heat-resistance superalloys are a group of materials engineered to have very high strength and superb corrosion resistance. These alloys must also preserve these properties at very high temperatures and chemically hostile environments. They are mainly used in jet engines, turbines, oil&gas equipment, and medical implants.

Introduction

Super Alloys ISO S

Heat-resistance superalloys are divided into three sub-categories defined according to the primary alloying element. They all share excellent heat and corrosion resistance, but each sub-group is better or worse in specific properties and used accordingly in different applications.

Many of the superalloys are Proprietary names owned by a handful of steel manufactures that develop these materials. Besides the chemical composition, the manufacturers guarantee all the material’s mechanical and physical properties in a wide range of temperatures.

Leading manufactures of heat resistance superalloys (HRSA):

Each alloy’s hardness is part of its specifications and is one of the main factors affecting machinability. The range is extensive, but many popular superalloys come in the range of 32-45 HRC.

Another critical factor is the low thermal conductivity that causes most of the heat to “stay” in the cutting zone instead of being absorbed by the chips and the workpiece.

Machinability of Nickel (Ni) based superalloys

Nickel-based superalloys are the most widely used alloys in this group. The main feature is excellent strength across a wide temperature range combined with good corrosion resistance. HRSA’s are used mainly for aircraft jet-engine parts and in the oil & gas industry. The machinability ranges between 9% to 45% depending mostly on the hardness of the material. The hardness ranges between 5 to 44 HRC, but most commonly used materials have a hardness of 32-42 HRC. The most popular material in this sub-group is Inconel 718, which has a machinability of 10%.

Machinability of Iron (Fe) based superalloys

Iron-based superalloys are a more economical alternative to nickel-based alloys. They provide the same advantages but to a lesser extent and at a lower price. They are used mostly on less critical components that still require heat resistance properties. This subgroup’s most popular material is A-286, with a hardness of 25 HRC and 25% machinability rating.

Machinability of Cobalt (Co) based superalloys

Cobalt-based superalloys excel in their wear resistance and chemical stability in harsh and hot conditions. Therefore, they are mainly used in valves and fittings in an acidic environment and medical implants such as artificial hip joints. It is the most difficult to machine superalloy sub-group, generating very high wear on the cutting edges. The machinability ranges from 5% up to 20%. The most popular materials in this sub-group are Stellite 6 & 21 with a 32-36 HRC hardness and machinability of around 18%.

Boosting machinability of superalloys with high-pressure coolant.

Since heat at the cutting edge is one of the biggest problems when machining superalloys, proper coolant implementation is crucial. To achieve the maximum effect, you need two steps:

  1. Use a tool with internal coolant delivery and an outlet as near as possible to the cutting edge, pointing directly at it. All the leading tool suppliers have dedicated tooling lines for this purpose.
  2. Add a high-pressure coolant pump to your machine with at least 70 bar (1000 PSI). Inconel 718 that can be turned with a regular tool and pump at a cutting speed of 140 SFM (45 mm/min), could be machined with a proper tool and pump at a cutting speed of 280 SFM (90 mm/min).

Boosting machinability of superalloys with SiALON ceramic inserts.

SiaLON is a silicon-nitride based ceramic cutting material combined with aluminum and oxides. It has excellent heat and wear resistance, making it an ideal option to machine HSRA alloys at much higher cutting speeds than conventional carbide inserts. For example, Inconel 718 that can be turned with a good carbide grade at a cutting speed of 140 SFM (45 mm/min), could be machined with a SiALON turning insert at a cutting speed of 700-800 SFM (240-250 mm/min)

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