Steel, stainless steel, cast iron, heat-resistant alloys... What is the difference between the cutting processes?

Metallic materials are divided into six main categories.

(1) P-steel

(2) M-Stainless steel

(3) K-Cast iron

(4) N-nonferrous metals

(5) S-Heat-resistant alloys

(6) H-hardened steel

Cutting characteristics of different materials

01-P steel

What is steel?

- Steel is the largest material group in the metal cutting industry.

- Steel can be unhardened or tempered (up to 400 HB hardness).

- Steel is an alloy with the iron (Fe) element as its main component. It is produced by a melting process.

- Unalloyed steels have a carbon content of less than 0.8% and contain only Fe and no other alloying elements.

- Alloy steels have a carbon content of less than 1.7%, with the addition of alloying elements such as Ni, Cr, Mo, V, W etc.

Within the metal cutting range, Group P is the largest material group, as it covers several different industrial areas.

The material is usually a long chip material, capable of forming a continuous, relatively uniform chip. The exact chip form is usually dependent on the carbon content.

- Low carbon content = tough, viscous material.

- High carbon content = brittle material.

Processing characteristics.

- Long chip material.

- Chip control is relatively easy and smooth.

- Mild steel is viscous and requires a sharp cutting edge.

- Unit cutting force kc: 1500-3100 N/mm2

- The cutting force and power required to machine ISO P materials are within a limited range of values.

02-M stainless steel

What is stainless steel?

- Stainless steel is an alloy material with a minimum of 11-12% chromium.

- The carbon content is usually very low (down to a maximum of 0.01%).

- The main alloys are Ni (nickel), Mo (molybdenum) and Ti (titanium).

- A dense layer of Cr2O3 forms on the surface of the steel, making it resistant to corrosion.

Most of the applications in Group M are in the oil and gas, pipe fittings, flanges, process industries, and pharmaceutical industries.

The material forms irregular flakes of chips, which give it a higher cutting force than regular steel. There are many different types of stainless steel. Chip breaking properties (ranging from easy to almost impossible to break chips) vary depending on alloy properties and heat treatment.

Machining characteristics.

- Long chip material.

- Chip control is relatively smooth in ferrite, difficult in austenitic and duplex.

- Unit Cutting Force: 1800-2850 N/mm2

- High cutting forces, chip lumps, heat and work hardening during machining.

03-K Cast Iron

What is cast iron?

- There are three main types of cast iron: gray cast iron (GCI), ductile iron (NCI) and graphite cast iron (CGI).

- Cast irons have a predominantly Fe-C composition, with a relatively high silicon content (1-3%).

- The carbon content exceeds 2%, which is the maximum solubility of C in the austenitic phase.

- Cr (chromium), Mo (molybdenum) and V (vanadium) are added to form carbides, which increase strength and hardness, but reduce machinability.

Group K is mainly used in the automotive parts, machine building and iron making industries.

The chip formation of the material varies from near-powdered chips to long chips. The power required to machine this material group is generally low.

Note that there is a significant difference between gray cast iron (which usually has chips that approximate powder) and ductile cast iron, which has chips that are more similar to steel in many cases.

Machining characteristics.

- Short chip material.

- Good chip control in all operating conditions.

- Unit cutting force: 790-1350 N/mm2.

- Machining at higher speeds results in abrasive wear.

- Moderate cutting forces.

04-N Non-Ferrous

What are non-ferrous materials?

- This category contains non-ferrous soft metals with hardness below 130 HB.

- Non-ferrous (Al) alloys with nearly 22% silicon (Si) make up the largest part of this category.

- Copper, bronze, brass.

The aircraft industry and aluminum automotive wheel manufacturers dominate Group N.

Although low power per mm3 (cubic inch) is required, it is still necessary to calculate the maximum power required to achieve high metal removal rates.

Processing Characteristics.

- Long chip material.

- If alloyed, chip control is relatively easy.

- Non-ferrous metals (Al) are viscous and require a sharp cutting edge.

- Unit cutting force: 350-700 N/mm2

- The cutting force and power required to machine ISO N materials are within a limited range of values.

05-S Heat-resistant alloy

What is a heat resistant alloy?

- Heat Resistant Alloys (HRSA) include many high alloyed iron, nickel, cobalt or titanium based materials.

- Group: iron-based, nickel-based, cobalt-based

- Conditions: annealed, solution heat treated, aged, rolled, forged, cast.

Characteristics.

- The higher alloy content (cobalt over nickel) ensures better heat resistance, higher tensile strength and higher corrosion resistance.

Difficult-to-machine Group S materials are mainly used in the aerospace, gas turbine and generator industries.

A wide range is available, but high cutting forces are often present.

Machining characteristics.

- Long chip material.

- Difficult chip control (jagged chips).

- Negative front angles are required for ceramics and positive front angles are required for carbides.

- Unit cutting force.

For heat-resistant alloys: 2400-3100 N/mm2

For titanium alloys: 1300-1400 N/mm2

- Requires high cutting forces and power.

06-H hardened steel

What is hardened steel?

- Hardened steels are the smallest grouping from a processing perspective.

- This grouping contains hardened steels with a hardness of >45-65 HRC.

- Typically, the hardened parts being turned have a hardness range of 55-68 HRC.

The hardened steels in Group H are used in a variety of industries, such as the automotive industry and its subcontractors, as well as machine building and tooling operations.

It is usually a continuous, red-hot incandescent chip. This high temperature helps reduce kc1 values and is important to help solve application challenges.

Process Characteristics.

- Long chip material.

- Relatively good chip control.

- Negative rake angle required.

- Unit cutting force: 2550-4870 N/mm2.

- The cutting force and power required is high.