Nov 7 2001
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 440C stainless steels are high carbon steels, which attain the highest hardness, wear resistance and strength of all stainless steel grades after heat treatment. These properties make this grade suitable for applications such as valve components and ball bearings. Grade 440A and 440B stainless steels, on the other hand, have similar properties - except for a slightly lower percentage of carbon in grade 440A.
All three forms of grade 440 steels are commonly used. However, grade 440C is more readily available than the other standard grades. Grade 440F, a free-machining type of grade 440 series, is also available with a high carbon content similar to that of grade 440C. Martensitic steel grades are high-hardness steels, usually fabricated using techniques that require hardening and tempering treatments.
These grades have a corrosion resistance lower than that of other austenitic grades. The applications of martensitic steels are limited by the loss of strength caused by over-tempering at high temperatures, and loss of ductility at temperatures below zero.
Key Properties
The following section discusses the properties of grade 440 bar products covered under ASTM A276. These values may not be similar to that of other forms such as forgings and plates.
Composition
The chemical compositions of various elements of grade 440 stainless steels are tabulated below:
Table 1. Chemical composition ranges of grade 440 stainless steels
|
Grade
|
C
|
Mn
|
Si
|
P
|
S
|
Cr
|
Mo
|
Ni
|
N
|
|
440A
|
min.
max.
|
0.6
0.75
|
-
1
|
-
1
|
-
0.04
|
-
0.03
|
16
18
|
-
0.75
|
-
|
-
|
|
440B
|
min.
max.
|
0.75
0.95
|
-
1
|
-
1
|
-
0.04
|
-
0.03
|
16
18
|
-
0.75
|
-
|
-
|
|
440C
|
min.
max.
|
0.95
1.20
|
-
1
|
-
1
|
-
0.04
|
-
0.03
|
16
18
|
-
0.75
|
-
|
-
|
Mechanical Properties
Table 2. Mechanical properties of grade 440C stainless steels
|
Tempering Temperature (°C)
|
Tensile Strength (MPa)
|
Yield Strength 0.2% Proof (MPa)
|
Elongation (% in 50mm)
|
Hardness Rockwell (HR C)
|
Impact Charpy V (J)
|
|
Annealed*
|
758
|
448
|
14
|
269HB max#
|
-
|
|
204
|
2030
|
1900
|
4
|
59
|
9
|
|
260
|
1960
|
1830
|
4
|
57
|
9
|
|
316
|
1860
|
1740
|
4
|
56
|
9
|
|
371
|
1790
|
1660
|
4
|
56
|
9
|
* Annealed properties are typical for Condition A of ASTM A276# Brinell hardness is ASTM A276 specified maximum for annealed 440A, B and C.
Physical Properties
The following table outlines the physical properties of grade 440 stainless steels:
Table 3. Typical physical properties of grade 440 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
|
0-200°C
|
0-600°C
|
at 100°C
|
at 500°C
|
|
440A/B/C
|
7650
|
200
|
10.1
|
10.3
|
11.7
|
24.2
|
-
|
460
|
600
|
Grade Specification Comparison
Grade specifications for 440 stainless steels are given in the following table:
Table 4. Grade specifications of grade 440 stainless steels
|
Grade
|
UNS No
|
Old British
|
Euronorm
|
Swedish SS
|
Japanese JIS
|
|
BS
|
En
|
No
|
Name
|
|
440A
|
S44002
|
|
|
-
|
-
|
|
SUS 440A
|
|
440B
|
S44003
|
|
|
1.4112
|
X90CrMoV18
|
|
SUS 440B
|
|
440C
|
S44004
|
-
|
-
|
1.4125
|
X105CrMo17
|
-
|
SUS 440C
|
Possible Alternative Grades
Suitable alternatives to grade 440 stainless steels are listed in the table below:
Table 5. Possible alternative grades to 440 stainless steels
|
Grade
|
Reasons for choosing 440C
|
|
440A/B
|
Slightly softer and more corrosion resistant grade needed
|
|
440F
|
High machinability required, with same hardness and hardenability as 440C
|
|
420
|
Lower strength and hardness needed than any of the 440 grades
|
|
416
|
Higher machinability required, and the much lower hardness and strength is still adequate
|
Corrosion Resistance
Grade 440 stainless steels exhibit excellent resistance to mild acids, alkalis, foods, fresh water and air. A smooth polished surface also helps grade 440 steels resist corrosion in tempered, passivated and hardened conditions.
Grade 440C steels exhibit corrosion resistance similar to that of grade 304 steels.
Heat Resistance
Grade 440 stainless steels should not be used at temperatures below the relevant tempering temperature, due to loss of mechanical properties caused by over-tempering.
Heat Treatment
Annealing — Grade 440 stainless steels are full annealed at 850 to 900°C, followed by slow furnace-cooling at about 600°C and air-cooling. Sub-critical annealing is carried out at 735 to 785°C, followed by slow furnace-cooling.
Hardening — Grade 440 stainless steels are heated at 1010 to 1065°C, then quenched in air or oil. Oil quenching is usually carried out for heavy sections. Following this process, these steels are tempered at temperatures from 150 to 370°C, to achieve a high hardness and improved mechanical properties.
Tempering at temperatures between 425 and 565°C should be avoided as the corrosion and impact resistance properties of grade 440 tend to reduce in this range. Also, tempering of these grades at 590 to 675°C will result in high impact resistance and loss of hardness.
Welding
During welding, grade 440 stainless steels are pre-heated at 250°C, followed by full annealing. Grade 420 filler rods can be used to achieve a high hardness weld. Grade 309 or 310 filler rods will, however, provide soft welds, having high ductility.
Machining
Grade 440 stainless steels can be easily machined in their annealed state. Machining these grades after hardening is extremely difficult, or impossible in some cases.
Applications
Grade 440 stainless steels find applications in the following:
-
Chisels
-
Surgical equipment
-
High quality knife blades
-
Valve seats
-
Rolling element bearings