Background

Why we use Grade 440C in all our knives here at Alaska Guide.
Grade 440C is capable of attaining, after heat treatment, the highest strength, hardness and wear resistance of all the stainless alloys. Its very high carbon content is responsible for these characteristics, which make 440C particularly suited to such applications as ball bearings and valve parts.

Lessor Grades identified as "440A and 440B" are identical except for slightly lower carbon contents (0.60 - 0.75% and 0.75 - 0.95% respectively); these have lower attainable hardnesses but slightly higher corrosion resistances. Although all three versions of this grade are standard grades, in practice 440C is more available than the A or B variants.

A free-machining variant 440F (UNS S44020) also exists, with the same high carbon content as 440C. Again this grade is not readily available in Alaska.

Martensitic stainless steels are optimised for high hardness, and other properties are to some degree compromised. Fabrication must be by methods that allow for poor weldability and usually also allow for a final harden and temper heat treatment. Corrosion resistance is lower than the common austenitic grades, and their useful operating temperature range is limited by their loss of ductility at sub-zero temperatures and loss of strength by over-tempering at elevated temperatures.

Key Properties

These properties are specified for bar product in ASTM A276. Similar but not necessarily identical properties are specified for other products such as plate and forgings in their respective specifications.

Composition

Table 1. Composition ranges for 440 grade stainless steels.

Grade

C

Mn

Si

P

S

Cr

Mo

Ni

N

440A

min.

max.

0.65

0.75

-

1.00

-

1.00

-

0.040

-

0.030

16.00

18.00

-

0.75

-

-

440B

min.

max.

0.75

0.95

-

1.00

-

1.00

-

0.040

-

0.030

16.00

18.00

-

0.75

-

-

440C

min.

max.

0.95

1.20

-

1.00

-

1.00

-

0.040

-

0.030

16.00

18.00

-

0.75

-

-

Mechanical Properties

Table 2. Mechanical properties of 440C grade 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

Table 3. Typical physical properties for 440 grade stainless steels.

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

0-200°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

Table 4. Grade specifications for 440 grade 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

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

Table 5. Possible alternative grades to 440 stainless steel.

Grade

Why it might be chosen instead of 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

Good resistance to the atmosphere, fresh water, foods, alkalies and mild acids. Best resistance in the hardened and tempered and passivated condition. A smooth polished surface also assists.

The corrosion resistance of grade 440C approximates that of grade 304 in many environments.

Heat Resistance

Not recommended for use in temperatures above the relevant tempering temperature, because of reduction in mechanical properties by over-tempering.

Heat Treatment

Annealing - Full anneal - 850-900°C, slow furnace cool to about 600°C and then air cool. Sub-critical Annealing - 735-785°C and slow furnace cool.

Hardening - Heat to 1010-1065°C, followed by quenching in warm oil or air. Oil quenching is necessary for heavy sections. Immediately temper at 150-370°C to obtain a wide variety of hardness values and mechanical properties as indicated in the accompanying table.

Tempering in the range 425-565°C is to be avoided because of reduced impact resistance and corrosion resistance. Tempering in the range 590-675°C results in lower hardness (the product become machinable) and high impact resistance.