In metal cutting processes, there are different metal work materials, and each material has unique chip formation and removal characteristics. How can we understand the properties of different materials? ISO standards classify metal materials into 6 groups, each with distinct machinability properties. This article will summarize them separately. Metal materials are classified into 6 groups: (1) P – Steels (2) M – Stainless steel (3) K – Cast irons (4) N – Nonferrous metals (5) S – Heat-resistant alloys (6) H – Hardened steels
What is steel?
– Steel is the largest material group in metal cutting. – Steel can be non-hardened or tempered steel (up to 400HB hardness). – Steel is an alloy with iron (Fe) as the main element. It is produced via smelting processes. – Non-alloy steels have carbon content below 0.8%, containing only Fe without other alloying elements. – Alloy steels have carbon content below 1.7% and contain alloying elements like Ni, Cr, Mo, V, and W. Within metal cutting, Group P is the largest material group, covering several industrial sectors. Materials are typically long chip materials, forming continuous, relatively uniform chips. Specific chip shape depends on carbon content. – Low carbon = Tough, gummy materials. – High carbon = Brittle materials. Machining characteristics: – Long chip materials. – Chips are relatively easy to control and manage. – Low-carbon steels are gummy and need sharp cutting edges. – Specific cutting force kc: 1500–3100 N/mm2 – Cutting forces and power required to machine ISO P materials are within moderate ranges.
What is stainless steel?
– Stainless steels are alloy materials with a minimum of 11-12% chromium. – Carbon content is typically very low (as low as max 0.01%). – Alloys are mainly Ni (nickel), Mo (molybdenum), and Ti (titanium). – A dense Cr2O3 layer forms on the steel surface, giving corrosion resistance. Materials form irregular, chip-like chips, with higher cutting forces than regular steels. Many different types of stainless steel exist. Chip breakability (from easy to almost unbreakable) varies with alloy characteristics and heat treatment. Machining characteristics: – Long chip materials. – Chips are relatively easy to control in ferritic grades, more difficult in austenitic and duplex. – Specific cutting force: 1800-2850 N/mm2 – Generate high cutting forces, build-up edges, heat, and work hardening during machining.
What is cast iron?
– Three main types of cast irons: gray cast iron (GCI), nodular cast iron (NCI), and compacted graphite iron (CGI). – Cast irons have Fe-C as main components, with relatively high silicon content (1-3%). – Carbon content above 2%, the maximum solubility of C in austenite phase. – Cr (chromium), Mo (molybdenum), and V (vanadium) are added from carbides, increasing strength and hardness but decreasing machinability. Chip formation varies, from near powdery chips to longer chips. The power required to machine this group is usually modest. Note significant differences between gray cast iron (usually powdery chips) and nodular cast iron, which often chips more like steel. Machining characteristics: – Short chip materials. – Good chip control under all conditions. – Specific cutting force: 790-1350 N/mm2 – Abrasive wear at higher speeds. – Moderate cutting forces.
What are nonferrous metal materials?
– This group contains nonferrous metals and soft metals with hardness below 130 HB. – Aluminum (Al) alloys with close to 22% silicon (Si) make up the largest part. – Copper, bronze, brass. Although the power required per mm3 (cubic inch) is low, maximum power must still be calculated for high metal removal rates. Machining characteristics: – Long chip materials. – Chips are relatively easy to control if alloys. – Aluminum (Al) is gummy, needing sharp cutting edges. – Specific cutting force: 350-700 N/mm2 – Cutting forces and power required to machine ISO N materials are within limited ranges.
What are heat-resistant alloys?
– Heat-resistant alloys (HRSA) include many high alloy iron, nickel, cobalt or titanium-based materials. Groups: Iron-based, nickel-based, cobalt-based Conditions: Annealed, solution heat treated, aged, rolled, forged, cast. Characteristics: – Higher alloy content (cobalt over nickel) ensures better heat resistance, higher tensile strength, and corrosion resistance. Machining characteristics: – Long chip materials. – Difficult chip control (serrated chips). – Negative rake angles are needed for ceramics, and positive for carbides. – Specific cutting force: For heat-resistant alloys: 2400–3100 N/mm2 For titanium alloys: 1300-1400 N/mm2 – High cutting forces and power required.
What are hardened steels?
– From a machining perspective, hardened steels are the smallest group. – The group contains tempered steels with hardness >45-65 HRC. – Typically hardness of hardened parts being turned is in the 55-68 HRC range. Usually continuous, red hot chips. The high temperature helps lower the kc1 value, which is important to aid in application challenges. Machining characteristics: – Long chip materials. – Relatively good chip control. – Require negative rake angles. – Specific cutting force: 2550-4870 N/mm2 – High cutting forces and power required.
What machining method is mainly used for group P steels?
Group P steels are long chip materials, so continuous cutting is generally used.
What should be noted when machining stainless steel?
Stainless steel chips are not easy to break, so chip length should be controlled. Also, pay attention to chip wear on the tool.
How to obtain the best cutting results for cast irons?
Cast irons are short-chip materials, so suitable cutting parameters should be selected to make powdery chips for the best results.
What is the most suitable cutting method for nonferrous metals?
Nonferrous metals are also long-chip materials, so high-speed continuous cutting can achieve high efficiency.
What factors should be considered when machining heat-resistant alloys?
Heat-resistant alloy chips are hard to control, so proper tool geometry should be selected to obtain optimum chip shapes.
Any suggestions for machining hardened steels?
Hardened steels have large cutting forces, so negative rake tools should be selected, and attention paid to selecting cutting parameters.