Bright heat treatment using hydrogen atmosphere

Equilibrium temperature and dew point of metal oxides in a hydrogen atmosphere

Equilibrium temperature and dew point of metal oxides in a hydrogen atmosphere

Generally, heat treatment exposes the target work to temperatures higher than the transformation point of the metal, causing the work’s surface to combine with the moisture and oxygen in the air, resulting in oxidation.
Heating and cooling the work in a protective atmosphere enables performing the heat treatment while maintaining the metal surface texture. For stainless steel, a more advanced technique treats the work in a reducing atmosphere of hydrogen, hydrogen and nitrogen, or ammonia cracking atomsphere gas to reduce any oxides on the surface, creating a bright and glittery silver white treatment.

The graph on the left shows the equilibrium temperature and dew point of metal oxides in a hydrogen atmosphere.
The dew point is the temperature at which moisture in the air condenses, which can be converted to the moisture content in the air. To produce the bright treatment, controlling the dew points below the red line in the graph that indicates the oxidation and reducing the chromium that characterizes stainless steel is essential.

Relationship between the carbon content and sensitization of austenitic stainless steel SUS 304

Relationship between the carbon content and sensitization of austenitic stainless steel SUS 304

The graph on the left shows the relationship between the carbon content and sensitization of austenitic stainless steel SUS 304.
A high cooling speed is required to prevent chromium carbides from precipitating on the grain boundary.

Target works (typical examples)

  • Annealing for reducing stainless steel hardness
    Examples: Flexible pipes, pipes for automobile components, watch parts, bellows, pressed parts
  • Solution heat treatment for removing residual stresses and work hardening to improve workability and corrosion resistance
    Examples: Stainless steel pipes, tableware, stainless steel watchbands, pressed parts
  • Quenching for increasing stainless steel hardness
    Examples: Kitchen knives, scissors, disc brakes, tools, cutting tools, micro-shafts for small motors
  • Brazing for joining base materials by melting a brazing material that has a lower melting point than the base materials and diffusing it through capillarity action
    Examples: Heat exchangers, automobile parts, hydraulic equipment, gas appliances, building parts, farm equipment parts, stainless steel filters

Chemical composition, properties and applications of stainless steel materials

(Main materials of austenitic stainless steel) JIS

JIS symbol Chemical composition (%) Solution heat treatment Tensile strength Elongation Hardness Applications
C Si Mn Ni Cr Mo Other (℃) kg/mm2 % HB
SUS304 ≤0.08 ≤1.00 ≤2.00 8.0 to 10.5 18.0 to 20.0 - - 1010 to 1150 Quick cooling ≥53 ≥40 ≤187 Standard type, general corrosion resistant applications
SUS310S ≤0.08 ≤1.50 ≤2.00 19.0 to 22.0 24.0 to 26.0 - - 1030 to 1180 Quick cooling ≥53 ≥40 ≤187 Heat resistant, oxidation resistant applications
SUS316 ≤0.08 ≤1.00 ≤2.00 10.0 to 14.0 16.0 to 18.0 2.0 to 3.0 - 1010 to 1150 Quick cooling ≥53 ≥40 ≤187 Standard type, general corrosion resistant applications
SUS321 ≤0.08 ≤1.00 ≤2.00 9.0 to 19.0 17.0 to 19.0 - Ti5 x C% or more 950 to 1150 Quick cooling ≥53 ≥40 ≤187 Grain boundary corrosion resistant type, welding applications in chemical industry

(Main materials of martensitic stainless steel) JIS

JIS symbol Chemical composition (%) Heat treatment(℃) Tensile strength Elongation Hardness HB Applications
C Si Mn Ni Cr Mo Quenching Tempering kg/mm2 % Quenching Tempering Annealing
SUS403 ≤0.15 ≤0.50 ≤1.00 ≤0.60 11.5 to 13.0 - 950 to 1000 Oil cooling 700 to 750 Quick cooling ≥60 ≥40 ≥170 ≤183 Pump shafts, turbine blades
SUS420J2 0.26 to 0.40 ≤1.00 ≤1.00 ≤0.60 12.0 to 14.0 - 920 to 980 Oil cooling 600 to 750 Quick cooling ≥75 ≥40 ≥217 ≤235 Cutting tools, micro-shafts for audio equipment
SUS420F 0.26 to 0.40 ≤1.00 ≤1.25 ≤0.60 12.0 to 14.0 ≤0.60 920 to 980 Oil cooling 600 to 750 Quick cooling ≥75 ≥40 ≥217 ≤235 Free-cutting property, cutting tools, shafts
SUS440C 0.95 to 1.20 ≤1.00 ≤1.00 ≤0.60 16.0 to 18.0 ≤0.75 1010 to 1070 Oil cooling 100 to 180 Quick cooling - - ≥58 ≤269 Cutting tools, pump parts, bearings

(Main materials of ferritic stainless steel) JIS

JIS symbol Chemical composition (%) Annealing Tensile strength Elongation Hardness Applications
C Si Mn Ni Cr Mo (℃) kg/mm2 % HB
SUS405 ≤0.08 ≤1.00 ≤1.00 ≤0.60 11.5 to 14.5 - 780 to 830 Air or slow cooling ≥42 ≥20 ≤183 Turbine blades, general applications
SUS430 ≤0.12 ≤1.75 ≤1.00 ≤0.60 16.0 to 18.0 - 780 to 850 Air or slow cooling ≥46 ≥22 ≤183 Furnace parts for up to 800℃
SUS434 ≤0.12 ≤1.00 ≤1.00 ≤0.60 16.0 to 18.0 0.75 to 1.25 780 to 850 Air or slow cooling ≥46 ≥22 ≤183 Applications in chemical industry

Advantages of continuous bright heat treatment

Although vacuum and batch furnaces are sometimes used for bright heat treatment, productivity is low in vacuum furnaces, and sufficient brightness cannot be produced in batch furnaces due to low substitution performance.
Bright heat treatment in vacuum furnaces causes metal molecules in the work to evaporate and deposit, which may lead to problems such as surface texture deterioration and failure to form surface oxide films.
In contrast, continuous furnaces provide high productivity and also enable easy substitution for better brightness.

Thermal products in this category

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