In metal cutting, there are different workpiece materials. Different materials have different cutting formation and removal characteristics. How can we master the characteristics of different materials? ISO standard metal materials are divided into 6 different types of groups. Each type has unique characteristics in terms of machinability. This article will summarize them separately.
Metal materials are divided into 6 major 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
01-P steel
What is steel? - Steel is the largest material group in the field of metal cutting. - Steel can be non-hardened steel or quenched and tempered steel (hardness up to 400HB). - Steel is an alloy with iron (Fe) as the main component. It is manufactured through a smelting process. - Non-alloy steel has a carbon content of less than 0.8%, only Fe and no other alloying elements. - Alloy steel has a carbon content of less than 1.7%, and alloying elements such as Ni, Cr, Mo, V, W, etc. are added.
Application areas: Within the metal cutting range, the P group is the largest material group, as it covers several different industrial sectors.
The materials are usually long-chipping materials that form continuous, relatively uniform chips. The specific chip form usually depends on the carbon content. – Low carbon content = tough, sticky material. – High carbon content = brittle material.
Machining characteristics: - Long-chipping material. - Chip control is relatively easy and smooth. - Mild steel is sticky and requires a sharp cutting edge. - Unit cutting force kc: 1500~3100 N/mm² - The cutting force and power required to process ISO P materials are within a limited range.
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 alloy is mainly Ni (nickel), Mo (molybdenum) and Ti (titanium). - A dense layer of Cr2O3 is formed on the surface of the steel, making it corrosion-resistant.
Applications: In group M, most applications are in the oil and gas, pipe fittings, flanges, processing industry and pharmaceutical industry.
Chip shape: The material forms irregular flake chips, which have higher cutting forces than ordinary steels. There are many different types of stainless steel. The chip breaking performance (from easy to almost impossible to break chips) varies depending on the alloy characteristics and heat treatment.
Machining characteristics: - Long chipping material. - Chip control is relatively smooth in ferrite, more difficult in austenite and duplex. - Specific cutting force: 1800~2850 N/mm² - High cutting forces, built-up edge, heat and work hardening are generated during machining.
03-K Cast Iron
What is cast iron? - There are 3 main types of cast iron: grey cast iron (GCI), ductile iron (NCI) and compacted graphite iron (CGI). - Cast iron is mainly composed of Fe-C with a relatively high silicon content (1%~3%). - The carbon content is more than 2%, which is the maximum solubility of C in the austenite phase. - Cr (chromium), Mo (molybdenum) and V (vanadium) are added to form carbides, which increase strength and hardness, but reduce machinability.
Application areas: K group is mainly used in automotive parts, machine manufacturing and ironmaking.
Chip shape: The chip shape of the material varies, from nearly powdery chips to long chips. The power required to process this material group is generally less. *Note: There is a big difference between grey cast iron (usually the chips are nearly powdery) and ductile iron, the chip breaking of the latter is often more similar to steel.
Machining characteristics: - Short-chipping material. - Good chip control under all working conditions. - Unit cutting force: 790~1350 N/mm²- Abrasive wear occurs when machining at higher speeds. - Medium cutting force.
04-N Nonferrous Metals
What are nonferrous materials? - This category includes nonferrous metals, soft metals with a hardness below 130 HB. - Nonferrous (Al) alloys containing nearly 22% silicon (Si) make up the largest part. - Copper, bronze, brass.
Application areas: Aircraft manufacturing and aluminum alloy automobile wheel manufacturers dominate the N group.
Although the power required per cubic inch is low, the maximum power required must still be calculated to achieve a high metal removal rate.
Machining characteristics: - Long chipping material. - In the case of alloys, chip control is relatively easy. - Nonferrous metals (Al) are sticky and require a sharp cutting edge. - Unit cutting force: 350~700 N/mm²- The cutting force and power required to process ISO N materials are within a limited range.
05-S Heat Resistant Alloys
What are Heat Resistant Alloys? - Heat Resistant Alloys (HRSA) include many high alloy iron, nickel, cobalt or titanium based materials. Group: Iron based, nickel based, cobalt based Working conditions: Annealing, solution heat treatment, aging treatment, rolling, forging, casting. Features: Higher alloy content (cobalt than nickel) ensures better heat resistance, higher tensile strength and higher corrosion resistance
Application areas: Difficult to process S group materials are mainly used in aerospace, gas turbine and generator industries.
The range is wide, but high cutting forces are usually present.
Processing characteristics: - Long chipping materials. - Difficult chip control (serrated chips). - Negative rake angles are required for ceramics and positive rake angles for carbides. - Unit cutting force: For heat resistant alloys: 2400~3100 N/mm² For titanium alloys: 1300~1400 N/mm² - The required cutting force and power are very high.
06-H Hardened Steels
What are hardened steels? - Hardened steels are the smallest group from a machining point of view. - This group contains quenched and tempered steels with a hardness >45~65 HRC. - Typically, the hardness of hard parts being turned is in the range of 55~68 HRC.
Applications: Hardened steels in the H group are used in a variety of industries, such as the automotive industry and its subcontractors, as well as in the machine building industry and the mold business.
Chip shape: Usually continuous, red-hot chips. This high temperature helps to reduce the kc1 value and is important to help solve application problems.
Machining characteristics: - Long chipping materials. - Relatively good chip control. - Negative rake angle is required. - Unit cutting force: 2550~4870 N/mm² - The required cutting force and power are very high.
