ASTM E18
Standard Test Methods for Rockwell Hardness of Metallic Materials
Test methods for Rockwell and Rockwell superficial hardness of metallic materials — indenters, test forces, procedure, scale designations, and verification of testing machines and reference test blocks.
What ASTM E18 specifies
ASTM E18 is the method behind almost every hardness value on a heat-treated drawing. It defines the Rockwell scales as a combination of indenter and total force: the diamond spheroconical indenter with 150 kgf (HRC), 60 kgf (HRA) and 100 kgf (HRD); the 1/16 in (1.588 mm) tungsten-carbide ball with 100 kgf (HRB), 60 kgf (HRF) and 150 kgf (HRG); larger balls for the E, H and K scales; and the superficial scales with 15, 30 or 45 kgf total force (15N/30N/45N with diamond, 15T/30T/45T with the 1/16 in ball). It specifies the 10 kgf (3 kgf superficial) preliminary force, dwell times, a minimum specimen thickness of ten times the indentation depth for diamond scales (fifteen for ball scales), spacing of three diameters between indentations and two and a half from an edge, corrections for cylindrical surfaces, and daily indirect verification with standardised test blocks. Reported values carry the scale, for example 58 HRC or 88 HR15N.
ASTM E18 types and classes
Every designation the document defines, with the thickness it requires and the tests it imposes.
| Designation | Meaning | Requirement / test |
|---|---|---|
HRC | Diamond indenter, 150 kgf (1471 N) total force; hardened and case-hardened steel, titanium; valid range 20–70 HRC. | Daily verification with HRC test blocks |
HRA | Diamond indenter, 60 kgf (589 N); cemented carbides, thin hardened steel, shallow cases. | Daily verification with HRA test blocks |
HRB | 1/16 in (1.588 mm) carbide ball, 100 kgf (981 N); annealed and low-strength steel, copper alloys, aluminum; valid to 100 HRB. | Daily verification with HRB test blocks |
HRE | 1/8 in (3.175 mm) ball, 100 kgf; aluminum and magnesium castings, bearing metals. | Daily verification with HRE test blocks |
HR15N | Diamond indenter, 15 kgf (147 N); thin case-hardened and nitrided layers, thin hardened sheet; converts to HRC through ASTM E140. | Daily verification with HR15N test blocks |
HR30N | Diamond indenter, 30 kgf (294 N); carburized and nitrided cases too thin for HRC but thicker than HR15N requires. | Daily verification with HR30N test blocks |
HR30T | 1/16 in carbide ball, 30 kgf; thin sheet of softer metals (annealed steel, copper alloys, aluminum). | Daily verification with HR30T test blocks |
Tests and the drawing call-out
Tests the document imposes
- Indirect (performance) verification with certified test blocks at each scale used, daily or before use
- Direct verification of test forces, indenter geometry and depth-measuring system at fixed intervals
- Test block standardisation traceable to national standards
- Specimen preparation: flat, clean, rigidly supported surface; minimum thickness and spacing checks
- Cylindrical-surface correction from the E18 tables when the diameter is below the limit
How to cite it
Drawing call-out
HARDNESS 58–62 HRC PER ASTM E18 AT LOCATION SHOWN (thin cases: HARDNESS 88 HR15N MIN PER ASTM E18)Angle brackets mark the choices the drawing must make; every finish page shows a completed example.
Heat treatments specified to ASTM E18
| Treatment | Role | Cost |
|---|---|---|
Age (Inconel 718) is a precipitation-hardening condition for Inconel 718 (UNS N07718) and Alloy 718 AM. | Test method | $$$$ |
Age C17200 (Beryllium Cu) is a precipitation-hardening condition for C17200 and C17300. | Test method | $$$ |
Age T6 (Aluminum) is a precipitation-hardening condition for 6061 and 6063 and 2024 and 7075. | Test method | $$ |
Age T651 (Aluminum) is a precipitation-hardening condition for 6061 plate and 2024 plate and 7075 plate. | Test method | $$ |
Age T73 (Aluminum) is a precipitation-hardening condition for 7075 and 7050 (alloy-specific overage condition). | Test method | $$$ |
Austempering forms bainite by quenching austenitized steel or ductile iron into a hot bath and holding isothermally. | Test method | $$$ |
Carbonitriding diffuses both carbon and nitrogen into low-carbon steel in the austenitic range, then quenches and tempers the surface to martensite. | Test method | $$ |
Carburizing diffuses carbon into low-carbon steel at austenitizing temperature, then quenches and tempers the enriched surface into a hard martensitic case. | Test method | $$$ |
Cryogenic treatment cools a previously hardened alloy below room temperature to transform retained austenite and improve dimensional stability. | Test method | $$ |
Flame hardening rapidly austenitizes selected steel surfaces with an oxy-fuel flame and immediately quenches them to martensite. | Test method | $$ |
High-temperature tempering reheats quenched steel around 500–650 °C to produce tempered martensite with high toughness and moderate hardness. | Test method | $ |
Induction hardening uses an alternating magnetic field to austenitize selected steel surfaces, followed immediately by quenching. | Test method | $$ |
Low-temperature tempering reheats freshly quenched steel at about 150–250 °C to relieve stress while retaining high martensitic hardness. | Test method | $ |
Martempering is an interrupted quench that equalizes austenitized steel just above its martensite-start temperature before cooling through martensite formation. | Test method | $$$ |
Medium-temperature tempering reheats quenched steel around 300–450 °C to trade some hardness for strength, elasticity, and improved toughness. | Test method | $ |
Q+T H1025 is a precipitation-hardening condition for 17-4 PH (S17400) and 15-5 PH (S15500). | Test method | $$ |
Q+T H1075 is a precipitation-hardening condition for 17-4 PH (S17400) and 15-5 PH (S15500). | Test method | $$ |
Q+T H1150 is a precipitation-hardening condition for 17-4 PH (S17400) and 15-5 PH (S15500). | Test method | $$ |
Q+T H900 is a precipitation-hardening condition for 17-4 PH (S17400) and 15-5 PH (S15500). | Test method | $$ |
Quench and temper transforms hardenable steel to martensite and then tempers it to the required balance of strength and toughness. | Test method | $$ |
Solution Treat + Age (Ti) is a precipitation-hardening condition for Ti-6Al-4V and Ti-6Al-6V-2Sn and Ti-10V-2Fe-3Al. | Test method | $$$$ |
Stress relief reduces residual stress from welding, machining, forming, casting, or additive manufacture without intentionally changing the bulk microstructure. | Test method | $ |
ASTM E18 — frequently asked
Which scale should I put on the drawing?
HRC for through-hardened and deep-case steel above about 20 HRC; HRB for annealed steel, copper and aluminum below that; HR15N or HR30N for thin cases and thin hardened sheet; HRE for aluminum castings. Do not specify HRC on a 0.2 mm nitrided case — the indenter senses the core and the reading is meaningless.
How thick must the part be?
At least ten times the indentation depth for diamond scales and fifteen times for ball scales. A 60 HRC reading indents about 0.08 mm and needs about 0.8 mm of material; a 20 HRC reading indents about 0.16 mm and needs about 1.6 mm. Thinner parts move to the superficial scales or to microhardness (ASTM E384).
Can I convert HRC to HB or HV?
Only approximately, through the ASTM E140 tables for the same material class; conversions are not acceptance values unless the specification allows them, and they are least reliable for cases, coatings and non-steel alloys. Specify the scale you can actually test on the part.
Why do my readings differ from the heat treater's?
Surface condition (scale, decarburization, curvature), specimen support and indenter or machine verification status account for most disputes. E18 requires a clean, flat, supported surface, applies corrections on cylinders below about 1 in (25 mm) diameter, and requires daily verification with test blocks; agree the test location and preparation on the drawing.
ASTM E18 is copyrighted by ASTM International. This page explains the document so a drawing can be read; it does not reproduce the standard. Purchase the current edition from the publisher. Official source ↗