When present in substantial amounts, Nickel provides a number of benefits to steel.
Nickel’s main contribution to steels is making them more forgiving of heat treatment variations. Think of it as the Heat Treater’s Friend.
Nickel lowers the critical temperatures, while widening the the temperature range for effective quenching and tempering. Nickel also retards the decomposiition of Austenite. Since nickel doesn’t form carbides, it doesn’t complicate the reheating for austenitizing process either. Nickel contributes to an easier and more likely to be successful heat treatment.
Here are 5 Contributions Nickel makes to our alloy steel parts:
Improved toughness (especially at low temperatures!)
Simplified and more economical heat treatment (Money saved!)
Increased hardenability (depth of hardness achievable)
Less distortion during quenching (more good parts after Q&T!)
Improved corrosion resistance (See this link– 2.1 % of GDP lost to corrosion!)
In addition to its appearance in the credits for 43XX, 46XX, and 86XX alloy steel grades, Nickel is a major component of Stainless Steels, Invar, Monel, and Inconel.
Machinist hint: When you see Nickel as a major ingredient in steel, avoid tool dwell and light cuts. Nickel contributes to a material’s workhardening ability. Photo credit.
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:
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.)
Poorer Plastic Forming than Coarse Grained Steels.
Less Distortion in Heat Treating than Coarse Grained Steels
Higher Ductility at the same hardness than Coarse Grained Steels
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.