Andrew Darley

Senior Technical Support

24 years engineering experience, specializing in milling, turning, grinding & EDM wire erosion.

At Cutwel, we speak to many people about the difficulties of machining Hardened Steel and how we would advise best to machine this particular material. In this article I have summarised all the common questions I get asked and also provided a bit of background information to help you on your way.

What is Hardened Steel?

Hardened Steel is a more wear-resistant and durable carbon steel that has been specially treated to offer improved hardness and strength over softer steels.

It is made by heating carbon steel to a higher temperature and then rapidly cooled. This process is called quenching. Because of quenching, the steel becomes extremely hard but also brittle. A solution to reducing brittleness without affecting the hardness, the material is reheated again and then left to naturally cool. This is called tempering. A combination of quenching and tempering results in Hardened Steel that can still be machined but without the risk of cracking.

What are the advantages and disadvantages of Hardened Steel?

Hardened Steel is extremely resistant to wear and abrasion. This is one of the main benefits when producing components that need to withstand regular abuse or heavy loads without failure or damage. It is also able to combat rust and corrosion better than standard steel.

Despite some of the pros of using Hardened Steel, it should be taken into consideration that despite its hardness it is unable to cope with sharp impacts. Also, you should be careful when subjecting the material to high temperatures due to the quenching and tempering processes which can lead to a reduced melting point.

What Rockwell is Hardened Steel?

Typically Hardened Steels rank on the Rockwell scale between 40HRC and 70HRC. Materials between 55-70HRC would generally be classed as high Hardened Steels. To compare all the different types of Steel and where Hardened Steels rank on the Rockwell scale, please see a summary below:

How to machine Hardened Steel and improve your processes

First off, it is not possible to use HSS or standard Carbide tooling when milling Hardened Steel, especially above 50HRC. We would always recommend a superior grade of carbide such as Ultrafine or Nanograin. These substrates are much denser which results in a harder-wearing tool with extremely high heat resistance.

Once you have selected the right type of tool you then need to consider what cutting data (speeds and/or feeds) you should use. Where possible, always follow the supplier or manufacturer’s recommended cutting data. If you are unsure of what speeds or feeds to use when machining Hardened Steel please do not hesitate to contact me or any member of the Cutwel technical team for more advice.

Traditionally, machining Hardened Steel has always been a slow and time-consuming process. This was down to low cutting speeds and feeds being used, plus there was always a risk of deep stepped tool marks being formed on the workpiece due to the large cutting depths used, particularly when roughing. EDM (Electric discharge machining) can also be used on Hardened Steel which could end up being a very time-consuming and costly process.

In the modern day, high-speed Hardened Steel tooling technologies now allow small cutting depths with large feed rates. These are increasingly becoming more prominent and replacing the older, more time-consuming methods.

To be successful when machining Hardened Steel, you must be aware of some key factors that can affect the process:

Chip Load

One major thing to look out for, which is key to maintaining tool life and machining quality, is maintaining a constant chip load on the tool’s cutting edge. In milling, for instance, the chip load equals feed rate divided by spindle speed, multiplied by the number of cutting flutes.

The chip load varies widely, if the load is too low or too high it will cause them to wear out too fast, chip or break. Lots of high speed and feed cuts with small depths is the best method for ensuring a quality product.

Reduced Tool Runout

Tool run-out is quite important when machining Hardened Steel because if the runout is greater than 0.01mm, the tool life can be greatly reduced. This is particularly relevant on some small-diameter tools where the chip load can also be doubled if the runout is too big. This makes choosing the right toolholder for your application an absolute priority. High-precision chucks negate the impact of tool runout. Please contact me or the Cutwel Technical Team for advice on choosing the right toolholder for machining Hardened Steel.

Tool Life

Managing tool life is always important and when machining Hardened Steel you may experience high cutting temperatures which can reduce how long a tool will optimally perform. We recommend using small cut depths when machining Hardened Steel because of the increased time the tool is exited from the cutting process. It results in the cutting edge being cooled, thus increasing its tool life.

Also on harder materials, thermal shock can occur, therefore air jet cooling, oil mist or air mist can be used. Jet cooling is the best option however as it eliminates any requirements for the tool to withstand rapid and severe temperature changes.

Selecting the Right Tool

In this section, I will advise in general what to consider when choosing the correct tools for Hardened Steel machining. However, if you require specific tooling recommendations for dedicated Hardened Steel machining then please see here.

Due to the heat produced when machining Hardened Steel, you will require tools with a thermal barrier coating. In addition to this, cobalt content can also improve heat resistance and strength. Negative rake angle tools should also be adopted to protect the cutting edge against the hardness of the material.

CBN (Cubic Boron Nitride) is the 2nd hardest material, only surpassed by diamond, therefore CBN tooling is also commonly used in Hardened Steel applications due to its superior wear resistance and toughness.

What industries & applications are Hardened Steels mainly used in?

Hardened Steels are often seen in the automotive, bearing and mold & die industries.

For instance, in the automotive industry (as well as mechanical and plant engineering), Hardened Steel is often used to create parts such as gears, bolts, cardan shafts and coupling parts. The hardness and strength make the material have a high strength-to-weight ratio, which supports any applications that are subjected to shock loads or wear and tear.

In Mold and Die, Hardened Steel is commonly used to construct the molds themselves. Despite its higher cost, it is chosen over other materials (such as Aluminium) due to the longer life span which outweighs the initial investment over a larger number of parts.

How does Hardened Steel compare to other metals

Hardened Steel vs Steel

Hardened Steel has a higher hardness and tensile strength than normal steel due to the heat treatment process.

Hardened Steel vs Stainless Steel

The main advantage of Stainless Steel over Hardened Steel is its corrosion resistance. Particularly because Hardened Steel is prone to cracking.

Hardened Steel vs Exotics / HRSA’s

Similar to Stainless Steel, Exotics (or Heat Resistant Super Alloys) are preferred where corrosion resistance is required. Not only that but Exotics also have a higher temperature resistance than Hardened Steel.

Hardened Steel vs Cast Iron

Despite Hardened Steel potentially being brittle or prone to cracking, Cast Iron is particularly more so (except for malleable cast iron).

Cast Iron is still classed as a hard material however it is less ductile than Steel.

Hardened Steel vs Aluminium

Even without the hardening process, Steel is generally harder than Aluminium making it stronger and more durable.

However Aluminium is more malleable and elastic than Steel & Hardened Steel.

For the ultimate performance on Hardened Steel, look no further than YG-1’s flagship X5070 Blue milling cutters. Dedicated for ultra-high performance machining of Hardened Steel up to 70HRC, these cutters can cope with extreme heat up to 1,400°C. The nanograin carbide substrate combined with YG-1’s unique silicon-based coating means nothing cuts Hardened Steel faster or for longer than X5070 Blue. The range includes a wide range of sizes from just 0.1mm diameter and a host of geometries including long neck, corner radius, ball nose and 6-8 flute finishing tools.

4G Mills from YG-1 are ideal for Hardened Steels up 55HRC. This range boasts a huge selection of sizes including micro tools from just 0.03mm diameter and a huge range of geometries including standard, ball nose, 6 flute finishers and reduced neck options. Despite the majority of 4G mills being made from a premium micrograin carbide, the unique tool geometry and coating allow them to machine up to 1200°C with outstanding wear resistance making them suitable for Hardened Steel up to 55HRC. There are also a select few series made from a nanograin carbide substrate for higher-performance machining.

X- Power Pro milling cutters from YG-1 are exceptional for dry high-speed cutting of Hardened Steel. Made from an ultrafine micrograin carbide, X-Power Pro is ideal for many Hardened Steel applications such as mold & die, tool making or high-precision machining. The range includes micro diameters from just 0.4mm and has a range of geometries including standard, corner radius, long neck, ball nose and 6-8 flute finishing tools.

On the lower end of the budget, general-purpose coated carbide cutters such as K2 Carbide, NC Mills or Mammut Carbide can machine Hardened Steel up to 45HRC, but depending on your setup and machining capabilities, we would advise you to progress to a higher performing range such as 4G Mills or X-Power Pro to get the best results.

Ground CBN (cubic boron nitride) turning inserts for high performance machining of hardened steel.

Designed for ultra high performance machining of steel, HRSA's & hardened steels up to HRc70.

Carbide drills designed for high performance drilling of hardened steels up to HRc70.