Background

302HQ is a specialised wire grade finding very wide usage for manufacture of stainless steel fasteners. The inclusion of 3% copper in the composition reduces the cold work hardening rate substantially compared to Grade 304. This grade is the standard material for manufacture of self-tapping screws and light machine screws and is also used for some bolts, set screws, rivets and specialised fasteners. 302HQ has now totally replaced Grades 384 and 305. Alternative designations for Grade 302HQ include "XM-7", "304Cu" and "304HQ". The stable austenitic structure makes 302HQ non-magnetic, even after substantial cold work, and also results in excellent toughness, even down to cryogenic temperatures.

Key Properties

These properties are specified for wire in ASTM A493; wire is the only commonly available form for this grade. Composition Typical compositional ranges for grade 302HQ stainless steels are given in table 1. Table 1. Composition ranges for 302HQ grade stainless steel Grade C Mn Si P S Cr Mo Ni Cu 302HQ min. max. - 0.03 - 2.00 - 1.00 - 0.045 - 0.030 17.0 19.0 - 8.0 10.0 3.0 4.0 Mechanical Properties Typical mechanical properties for grade 302HQ stainless steels are given in table 2. Table 2. Mechanical properties of 302HQ grade stainless steel Grade Tensile Strength (MPa) max. Yield Strength 0.2% Proof (MPa) Elongation (% in 50mm) Hardness Rockwell B (HR B) Brinell (HB) 302HQ annealed 605 - - - - 302HQ lightly drawn 660 - - - - Above values from ASTM A493. Higher strengths can be produced by heavy cold work - this may be required for certain applications. Physical Properties Typical physical properties for annealed grade 302HQ stainless steels are given in table 3. Table 3. Physical properties of 302HQ grade stainless steel in the annealed condition Grade Density (kg/m3) Elastic Modulus (GPa) Mean Coefficient of Thermal Expansion (mm/m/°C) Thermal Conductivity (W/m.K) Specific Heat 0-100°C (J/kg.K) Electrical Resistivity (nW.m) 0-100°C 0-315°C 0-538°C at 100°C at 500°C 302HQ 8027 193 17.2 17.8 18.8 16.3 21.5 500 720 Grade Specification Comparison Approximate grade comparisons for 302HQ stainless steels are given in table 4. Table 4. Grade specifications for 302HQ grade stainless steel Grade UNS No Old British Euronorm Swedish SS Japanese JIS BS En No Name 302HQ S30430 394S17 - 1.4567 X3CrNiCu18-9-4 - SUS XM7 These comparisons are approximate only. The list is intended as a comparison of functionally similar materials not as a schedule of contractual equivalents. If exact equivalents are needed original specifications must be consulted. Possible Alternative Grades Possible alternative grades to grade 302HQ stainless steels are given in table 5. Table 5. Possible alternative grades to 302HQ grade stainless steel Grade Why it might be chosen instead of 302HQ 304L A higher work hardening rate can be tolerated - or is needed. 316L Higher resistance to pitting and crevice corrosion is required in chloride environments; the higher work hardening rate of 316L is acceptable. 430 A lower cost is required, and the reduced corrosion resistance of 430 is acceptable. Corrosion Resistance Excellent. Equal to or exceeding that of Grade 304 in a wide variety of corrosive media. Subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. Considered resistant to potable water with up to about 200mg/L chlorides at ambient temperatures, reducing to about 150mg/L at 60°C. Heat Resistance Good oxidation resistance in intermittent service to 870°C and in continuous service to 925°C. Continuous use of Grade 302HQ in 425-860°C range is usually safe (free of carbide precipitation) as the grade has a very low carbon content. Heat Treatment Solution Treatment (Annealing) - Heat to 1010-1120°C and cool rapidly. This grade cannot be hardened by thermal treatment. Welding Use Grade 308L rods or electrodes. Excellent weldability by all standard fusion methods, both with and without filler metals. Because of its applications this grade is not often welded. Exceptions are resistance butt welding to join wires together during wire manufacture, and when the grade is used to make stud welding fasteners. 302HQ is not specifically listed in AS 1554.6. Machining 302HQ is rarely machined, because of its form and likely products. The grade always has very low sulphur content as this aids formability, but unfortunately this also reduces machinability. Machining is certainly possible. An Improved Machinability version of Grade 302HQ is produced, having a very high machinability. This version has a slightly higher sulphur content and is also calcium treated. This Improved Machinability grade (referred to as Ugima 4567) is available only to special order. Cold Work Hardening 302HQ has the lowest work hardening rate of any of the common austenitic stainless steels. This results in a tensile strength increase of approximately 8MPa/%Ra (8MPa increase in tensile strength for each 1% reduction of area of cold work - this data from wire drawing). Even after substantial cold work this grade remains essentially non-responsive to a magnet.

Applications Typical applications include: · All severe cold heading applications · Self-tapping screws · Roofing bolts · Machine screws · Bolts · Set screws · Blind rivets