Nov 7 2001
Chemical Formula
Fe, <0.20% C, 15-17% Cr, 1.25-2.50% Ni, <1.00% Mn, <1% Si, <0.040% P, <0.03% S
Topics Covered
Introduction
Key Properties
Composition
Mechanical Properties
Physical Properties
Grade Specification Comparison
Possible Alternative Grades
Corrosion Resistance
Heat Resistance
Heat Treatment
Welding
Machining
Applications
Introduction
Grade 431 stainless steels are martensitic, heat-treatable grades with excellent corrosion resistance, torque strength, high toughness and tensile properties. All these properties make them ideal for bolt and shaft applications. These steels, however, cannot be cold-worked owing to their high yield strength, hence they are suitable for operations such as spinning, deep drawing, bending or cold heading.
Fabrication of martensitic steels is generally carried out using techniques that allow hardening and tempering treatments and poor weldability. The corrosion resistance properties of grade 431 steels are lower than that of austenitic grades. The operations of grade 431 are limited by their loss of strength at high temperatures, due to over-tempering, and loss of ductility at negative temperatures.
Key Properties
The specified properties are for bar products in ASTM A276. The properties may not be necessarily similar to other forms such as forgings and plates.
Composition
Table 1. Composition ranges of grade 431 stainless steels
|
Grade
|
C
|
Mn
|
Si
|
P
|
S
|
Cr
|
Ni
|
|
431
|
min.
max.
|
-
0.20
|
-
1
|
-
1
|
-
0.04
|
-
0.03
|
15
17
|
1.25
2.50
|
Mechanical Properties
Table 2. Mechanical properties of grade 431 stainless steels
|
Tempering Temperature (°C)
|
Tensile Strength (MPa)
|
Yield Strength 0.2% Proof (MPa)
|
Elongation (% in 50mm)
|
Hardness Brinell (HB)
|
Impact Charpy V (J)
|
|
Annealed *
|
862
|
655
|
20
|
285 max
|
-
|
|
204
|
1345
|
1055
|
20
|
388
|
50
|
|
316
|
1295
|
1035
|
19
|
375
|
53
|
|
427
|
1350
|
1080
|
19
|
388
|
#
|
|
538
|
1140
|
965
|
19
|
321
|
#
|
|
593
|
1015
|
770
|
20
|
293
|
64
|
|
650
|
960
|
695
|
20
|
277
|
84
|
Grade 431 is frequently stocked and supplied in "Condition T", with specified tensile strength of 850 - 1000MPa.
* Annealed tensile properties are typical for Condition A of ASTM A276; annealed hardness is the specified maximum. Grade 431 is only rarely stocked in annealed Condition A.
# Grade 431 should not be tempered between 425 and 600°C, due to the low impact resistance associated with this temperature range.
Physical Properties
Table 3. Typical physical properties of grade 431 stainless steels
|
Grade
|
Density (kg/m3)
|
Elastic Modulus (GPa)
|
Mean Coefficient of Thermal Expansion (μm/m/°C)
|
Thermal Conductivity (W/m.K)
|
Specific Heat
0-100°C (J/kg.K)
|
Electrical Resistivity (nΩ.m)
|
|
0-100°C
|
0-315°C
|
0-538°C
|
at 100°C
|
at 500°C
|
|
431
|
7800
|
200
|
10.2
|
12.1
|
-
|
20.2
|
-
|
460
|
720
|
Grade Specification Comparison
Table 4. Grade specifications of grade 431 stainless steels
|
Grade
|
UNS No
|
Old British
|
Euronorm
|
Swedish SS
|
Japanese JIS
|
|
BS
|
En
|
No
|
Name
|
|
431
|
S43100
|
431S29
|
57
|
1.4057
|
X17CrNi16-2
|
2321
|
SUS 431
|
Possible Alternative Grades
Table 5. Possible alternative grades to 431 stainless steels
|
Grade
|
Why it might be chosen instead of 431
|
|
410
|
Only a lower hardened strength is needed.
|
|
416
|
High machinability is required, and the lower hardened strength and lower corrosion resistance of 416 is acceptable.
|
|
440C
|
A higher hardened strength, or hardness, than can be obtained from 420 is needed.
|
Corrosion Resistance
Grade 431 stainless steels have considerable resistance to salt water, but they are less resistant to tropical water when compared to that of grade 316 steels. Grade 431 steels have overall corrosion resistance similar to, or slightly lower than, that of grade 304 steels.
Grade 431 steels with a smooth surface finish perform well in tempered and hardened conditions.
Heat Resistance
Grade 431 steels are resistant to scaling at temperatures of 925°C in intermittent conditions, and 870°C during continuous operations. In general, these steels are not to be used at temperatures above standard tempering temperatures, owing to loss of mechanical properties.
Heat Treatment
Full anneal — Full annealing cannot be performed on grade 431 steels. This grade gets hardened even during slow cooling.
Process anneal — Grade 431 steels are heated to 620 to 660°C and then air-cooled.
Grade 431 steels are generally hardened by heating at temperatures from 980 to 1065°C, holding for nearly ½ h, followed by oil or air quenching. Complex or hardened parts of grade 431 steels can be pre-heated to temperatures from 760 to 790°C and tempered, to improve their mechanical properties. Tempering of these steels at 425 to 600°C should be avoided, owing to the loss of impact toughness at this temperature range.
Welding
Welding of grade 431 stainless steels is difficult due to the chances of cracking. It is recommended to pre-heat the materials to 200 - 300°C before welding, and carry out post-weld heat treatment at 650°C. Welding can be performed using grade 410 filler rods, but ductile welds can be achieved using grades 308L, 309 or 310 steels.
Machining
Grade 431 steels can be easily machined in their annealed state. However, it is extremely difficult to machine these steels if they are hardened above 30HRC.
Applications
Typical applications of grade 431 stainless steels include the following:
-
Laboratory equipment
-
Marine systems
-
Beater bars
-
Pump and propeller shafts
-
Nuts and bolts