Stainless Steel – Grade 21-6-9 (UNS S21900)

Topics Covered

Introduction
Chemical Composition
Physical Properties
Mechanical Properties
Thermal Properties
Other Designations
Fabrication and Heat Treatment
Machinability
Forming
Welding
Heat Treatment
Forging
Annealing
Hardening
Applications

Introduction

Stainless steels are called as high-alloy steels. They have high corrosion resistance than other steels due to the presence of large amounts of chromium in the range of 4-30%. Based on their crystalline structure, stainless steels are divided into martensitic, ferritic and austenitic. They are divided into another group known as precipitation-hardened steels, which are a combination of martensitic and austenitic steels.

The following datasheet will provide more details about grade 21-6-9 stainless steel, which has excellent toughness and high temperature oxidation resistance.

Chemical Composition

The following table shows the chemical composition of grade 21-6-9 stainless steel.

Element Content (%)
Iron, Fe 64
Chromium, Cr 20
Manganese, Mn 9
Nickel, Ni 6
Silicon, Si 1
Nitrogen, N 0.23
Carbon, C 0.080
Phosphorous, P 0.060
Sulfur, S 0.030

Physical Properties

The physical properties of grade 21-6-9 stainless steel are given in the following table.

Properties Metric Imperial
Density 7.83 g/cm3 0.283 lb/in³

Mechanical Properties

The mechanical properties of grade 21-6-9 stainless steel are displayed in the following table.

Properties Metric Imperial
Tensile strength, ultimate 883 MPa 128000 psi
Tensile strength, yield (@0.2%) 745 MPa 108000 psi
Elongation at break (in 50 mm) 56% 56%
Modulus of elasticity (typical for stainless steel) 200 GPa 29000 ksi
Machinability (based on 100 machinability for AISI 1212 steel) 30 30

Thermal Properties

The following table outlines the thermal properties of grade 21-6-9 stainless steel.

Properties Metric Imperial
Thermal expansion co-efficient (@ 25-95°C/77-203°F) 16.7 µm/m°C 9.28 µin/in°F
Thermal conductivity (@-180°C/-292°C) 7.80 W/mK 54.1 BTU in/hr.ft².°F

Other Designations

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Other designations that are equivalent to grade 21-6-9 stainless steel include the following:

  • AMS 5561
  • AMS 5562
  • AMS 5656
  • ASTM A269 (XM-10)
  • ASTM A276 (XM-10)
  • ASTM A312 (XM-10)
  • ASTM A314 (XM-10)
  • ASTM A412 (XM-10)
  • ASTM A479 (XM-10)
  • ASTM A580 (XM-10)
  • ASTM A813 (XM-10)
  • ASTM A814 (XM-10)

Fabrication and Heat Treatment

Machinability

Grade 21-6-9 stainless steel is difficult is machine due to its high work hardening rate. Hence, ample cutting fluids, positive feeds and slow speeds are preferred to overcome work hardening.

Forming

The yield strength of grade 21-6-9 stainless steel is double that of other conventional stainless steels. This has to be taken into consideration while choosing required forming pressures and techniques.

Welding

Grade 21-6-9 stainless steel can be welded using SMAW, MIG and TIG methods. Post-weld annealing can be performed on these steels to improve corrosion resistance.

Heat Treatment

Grade 21-6-9 stainless steel can be stress-relieved at temperatures below 705°C (1300°F). However, stress relieving of this steel below (900 to 1250°F) would affect the mechanical properties of the steel.

Forging

Forging of grade 21-6-9 stainless steel can be performed at 927 to 982°C (1700 to 1800°F) followed by water or air quenching.

Annealing

Grade 21-6-9 stainless steel can be annealed at temperature ranging from 982 to 1177°C (1800 to 2150°F) and then air cooled.

Hardening

Grade 21-6-9 stainless steel is unresponsive to heat treatment. It can be hardened only through cold reduction.

Applications

The following are the major applications of grade 21-6-9 stainless steel:

  • Aircraft components
  • Steam and autoclave parts
  • Chemical and pollution control equipment


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