High Speed Steel (HSS)

High Speed Steel (HSS) is a highly alloyed tool steel specifically engineered to retain hardness and cutting ability at the elevated temperatures — typically 500–600°C — generated at the tool-workpiece interface during high-speed metal cutting operations. The defining property distinguishing HSS from all other tool steel categories is hot hardness retention: after heat treatment to HRC 62–70 at room temperature, HSS maintains effective cutting hardness of HRC 50–55 at 500–600°C service temperature. This capability is achieved through a precisely balanced alloy system containing tungsten (W) and/or molybdenum (Mo) that form very stable M6C carbides resisting dissolution and coarsening at elevated temperature, vanadium (V) forming extremely hard MC carbides (Vickers hardness ~2800 HV) providing abrasive wear resistance at the cutting edge, and chromium (Cr) providing hardenability and oxidation resistance. In cobalt-alloyed grades (M35, M42), cobalt dissolves in the steel matrix raising its melting point and inhibiting carbide coarsening at high temperature, further improving hot hardness beyond standard M2. Cold work tool steels (D2, SKD11) have higher room-temperature hardness (HRC 58–62) but lose hardness above 200°C, making them unsuitable for cutting applications above slow speeds. Hot work tool steels (H13) have good elevated-temperature strength but are designed for die applications, not cutting. HSS uniquely combines the hardness, wear resistance, toughness, and hot hardness retention needed for continuous metal cutting at productive speeds.

M2, M35, and M42 represent three successive performance levels in the molybdenum-series HSS family, differentiated primarily by cobalt content and the resulting hot hardness performance. M2 (AISI M2, JIS SKH51, GB W6Mo5Cr4V2, DIN 1.3343) is the universal general-purpose HSS with composition C 0.85%, W 6%, Mo 5%, Cr 4%, V 2%, no intentional cobalt, achieving HRC 63–65 after standard heat treatment with retained hot hardness of HRC 50–52 at 500°C. M2 accounts for over 60% of global HSS production and is the correct choice for: general-purpose twist drills, taps, reamers, and milling cutters for machining carbon steel, low-alloy steel, cast iron, aluminium, copper, and engineering plastics at standard cutting speeds; form tools and broaches for production machining of easily-machinable materials; and all applications where the economics of HSS (lower cost than cobalt grades, easy resharpening) are more important than maximum performance in difficult materials. M35 (AISI M35, JIS SKH55, DIN 1.3243) adds 5% cobalt to the M2 composition, raising achievable hardness to HRC 65–67 and improving hot hardness at 500–550°C by approximately 3–5 HRC points over M2. M35 is specified for: drilling and tapping austenitic stainless steel, duplex stainless steel, and cast iron with sand inclusions; form tools for medium-hardness alloy steels; gear cutting tools for alloy steel gears; and applications where M2 tools show premature wear but M42 cost premium is not justified. M42 (AISI M42, JIS SKH57, DIN 1.3247) contains 8% cobalt plus increased molybdenum (9.5%), achieving maximum hardness of HRC 67–70 and the highest hot hardness retention of conventional (non-PM) HSS. M42 is specified for: machining nickel-based superalloys (Inconel 718, Hastelloy), titanium and titanium alloys, cobalt-chrome alloys, hardened steels (HRC 35–50), high-silicon aluminium alloys, and any difficult-to-machine material generating extreme cutting temperatures where M2 and M35 fail by rapid edge softening. M42 costs approximately 30–50% more than M2 and is slightly more brittle, so its use is justified only when the material being machined genuinely requires the elevated hot hardness.

Ingot metallurgy (IM) and powder metallurgy (PM) represent two fundamentally different production routes for HSS with dramatically different microstructural quality and cutting tool performance characteristics. Ingot Metallurgy HSS (conventional / standard quality) is produced by electric arc furnace melting, vacuum degassing, casting into large ingots (typically 500–2000 kg), and extensive hot working by forging and rolling to break down the coarse as-cast carbide network. The critical limitation of IM-HSS is that HSS solidifies with a coarse eutectic carbide network (carbide particle size 20–60μm for M2, larger for high-alloy grades) that cannot be fully broken down by mechanical working — residual carbide banding is always present in IM-HSS, varying from acceptable in small cross-sections (diameter <25mm round bar) to pronounced in large cross-sections (diameter >75mm). This carbide banding causes: reduced transverse toughness (tool breakage susceptibility in the band direction), non-uniform wear across the cutting edge as alternating carbide-rich and carbide-depleted zones wear at different rates, and difficulty achieving fine surface finish when grinding complex tool geometries. Powder Metallurgy HSS (PM-HSS, Crucible CPM, Böhler-Uddeholm ASP, Carpenter Micro-Melt) is produced by gas or water atomisation of the molten HSS alloy into fine spherical powder particles (10–150μm diameter), each particle solidifying independently with an extremely fine, uniformly distributed carbide structure (carbide size 1–5μm) free of segregation. The powder is screened, packed into steel cans, and hot isostatically pressed (HIP) or hot extruded to full density, then forged and rolled to bar stock. PM-HSS provides: up to 50% improvement in transverse rupture strength versus IM-HSS of the same composition; uniform wear across the full cutting edge width; dramatically improved grindability (lower grinding forces, better wheel life, superior ground surface finish); and the ability to achieve high-vanadium compositions (V 3–5% in M3/2, M4, CPM Rex grades) impractical in IM-HSS due to severe vanadium carbide segregation during ingot solidification. PM-HSS costs 2–4× the price of equivalent IM-HSS grades and is specified for premium cutting tools where extended tool life, improved toughness, and superior surface finish justify the material cost premium — gear cutting tools, precision reamers, form tools, and end mills for difficult materials are the primary PM-HSS applications.

M2 HSS heat treatment requires precise temperature control and multiple steps to achieve the intended microstructure of tempered martensite with dissolved carbides providing maximum hot hardness and wear resistance. The complete standard heat treatment cycle for M2 is: Soft Annealing (840–870°C, hold 2–4 hours, slow furnace cool at ≤15°C/hour to 600°C, then air cool) achieves hardness ≤255 HBW for machining tool blanks, twist drill fluting, and tap thread grinding. Pre-heating is a critical step before hardening — M2 must be preheated in two stages: first at 450–500°C (equalising soak), then at 850–870°C (intermediate preheat) — to prevent thermal shock cracking from the extreme temperature gradient when transferring to the austenitising furnace. Austenitising (hardening) temperature for M2 is 1210–1230°C — significantly higher than any other tool steel category — held for 2–5 minutes at temperature (not per unit thickness as for conventional steels, as the high alloy content means carbide dissolution is time-sensitive within a narrow optimal temperature window). Austenitising temperature is the most critical parameter in M2 heat treatment: too low (below 1200°C) leaves excessive undissolved carbides, reducing as-quenched hardness below HRC 60; too high (above 1240°C) causes grain growth and incipient melting at grain boundaries, producing a brittle tool prone to early failure. Quenching is performed by interrupted oil quenching, salt bath quenching (565°C salt bath then air cool), or high-pressure gas quenching (for vacuum furnace processing) to arrest carbide precipitation from austenite and achieve full martensitic transformation. Tempering must begin within 30 minutes of quench completion to prevent cracking from thermal stress and unstable retained austenite — M2 requires a minimum of triple tempering at 540–560°C (2 hours each cycle) to convert retained austenite to martensite through the secondary hardening reaction and achieve the final specified hardness of HRC 63–65. Sub-zero treatment at −70 to −85°C between the first and second temper cycles further reduces retained austenite and is recommended for precision tools requiring maximum dimensional stability.

High Speed Steel is supplied in several product forms optimised for different tool production workflows. Round Bars are the primary product form for twist drills, taps, reamers, end mills, and cylindrical tool blanks, available in diameter range from 1.0mm (micro-drill blanks) through 300mm (large form tool and broach blanks), in standard lengths of 1m, 2m, and 3m, in annealed (≤255 HBW) or bright annealed condition for machining. Small diameters (1–6mm) are commonly supplied in coil or straightened rod form. Flat Bars (rectangular bars) in thickness from 3mm to 200mm and width from 10mm to 300mm serve as blanks for form tools, cut-off blades, shaper blades, and flat tool applications where the rectangular cross-section reduces raw material waste compared to round bar. Ground Flat Stock (GFS) is a precision-processed product form of particular importance for HSS — it consists of HSS flat bars that have been surface-ground on all four faces to tight dimensional tolerances (typically h6 or h7 per ISO 286-1: for a 20mm width, h6 = 20.000/−0.013mm, h7 = 20.000/−0.021mm) and straightness (typically ≤0.1mm per 300mm length). Ground flat stock is supplied in standard width and thickness combinations from 3×10mm to 25×200mm in M2 and M42 grades, and enables direct production of form tools, shaper cutters, and flat tool blanks without external grinding operations — the tool maker saws the flat stock to length, mills or grinds the tool profile, hardens and tempers, and applies final sharpening grinding. This eliminates the setup cost of grinding to dimension from hot-rolled flat bar and reduces raw material waste to the profile grinding stock only. Annular Ring Blanks (hollow cylinders, or tube form) for shell reamers, shell end mills, and annular broach elements are available in select diameters. Custom-forged billets for gear hobs, large broaches, and special form tools exceeding standard bar stock dimensions are available with production lead time of 30–45 days.