5 Ways Fine Austenitic Grain Size Affects Your Machine Shop

While Austenitic Grain Size is a result of chemistry (composition), the changes that it evokes in our process are a result of material structure and properties, not just the chemical ‘ingredients.’
Steel that is fully deoxidized and grain refined is more sound, less susceptible to cracking and distorting, and more easily controlled in heat treat. Well worth it in final performance compared to the machinist’s increased tooling costs.
 Here are 5 Ways Austenitic Fine Grained steels can affect your shop:

1. Poorer Machinability than Coarse Grained Steels. (The hard oxides and nitrides resulting from deoxidation and grain refinement abrade the edge of tools and coatings- this is one reason that you go through more tooling on Fine Grained Steels.)
2. Poorer Plastic Forming than Coarse Grained Steels.
3. Less Distortion in Heat Treating than Coarse Grained Steels
4. Higher Ductility at the same hardness than Coarse Grained Steels
5. Shallower Hardenability than Coarse Grained Steels.

Fine Austenitic Grain Size is a result of  DELIBERATELY ADDDING grain refining elements to a heat of steel. Because these grain refining elements have been added, the steel has a “Fine Austenitic Grain Size.”
In order to make steels with this Austenitic Fine Grained Structure, the steel is first deoxidized , (usually with  Silicon) and then Aluminum, or Vanadium or Niobium are added. Aluminum, Vanadium, and Niobium are called grain refiners.
 After  the Silicon has scavenged most of the Oxygen out of the  molten steel, the grain refiner is added. (In this post I’ll stick with Aluminum as the example.) The added Aluminum reacts with Nitrogen in the molten steel to form Aluminum Nitride particles. These tiny particles precipitate along the boundaries of the Austenite as well as with in the Austenite grains. This restricts the  growth of the grains.
Because the deoxidation and grain refinement  create hard abrasive oxide and nitride particles, they machine and process differently than coarse grained steels.
Fine Austenitic Grain Size appears on the material test report as an ASTM value of 5 or greater. Values of 5, 6, 7, 8, or “5 and finer”  indicate that  the material is Austenitic Fine Grained. Typically 7 or 8 was  reported for the Aluminum  Fine Grain steels that I certified.
The methods for determining Austenitic Grain Size are detailed in ASTM Standard E112, Standard Test Methods for determining Average Grain Size.
To get the Coarse Austenitic Grain Size Story, see our post here.