M398 Stainless Steel

M398 (BÖHLER M398 MICROCLEAN) - is a martensitic chromium steel produced by powder metallurgy. Due to its alloying concept, this stainless steel provides extremely high wear resistance and high corrosion resistance - the ideal combination for highly wear-resistant tools.

M398 steel is a tool stainless steel from the Bohler-Uddeholm concern. It is intended for plastic injection molding.

Alloying concept:

Increase in macrohardness due to the increase in the volume of primary carbide, consisting of:

Vanadium-rich MC carbides (VC ~3,000 HV);
Chromium-rich carbides M₇C₃ (Cr₇C₃ ~2,200 HV);

Properties

Extremely high wear resistance;
High and isotropic dimensional stability during heat treatment;
High hardenability and compressive strength with >60 HRc;
Good toughness;
Good corrosion resistance;
Good grindability;
High polishability to a shine.

These properties allow you to obtain:

Long tool life, therefore reduced downtime and maintenance costs;
High precision components;
Constant tool life.

Microclean technology (powder metallurgy method) ensures the most uniform distribution of carbides, which guarantees stability, performance and durability in difficult conditions. The homogeneous structure helps to increase the wear resistance of M398 steel while maintaining a high level of impact toughness. The fine microstructure also ensures its excellent grinding and polishing, which allows for a long-lasting sharp edge.

Resistance to the corrosive and abrasive effects of plastics ensures long tool life and consistent product quality. Bohler M398 steel is the best choice for high-performance molds in the plastics injection molding industry, helping to reduce downtime and improve overall production results.

Chemical composition

Table: chemical composition of steel grade M398
Chemical composition of steel grade M398
C	Cr	Mo	W	V	Mn	Si	Fe
2,7	20,0	1,0	0,7	7,2	0,5	0,5	Other

The carbon content in M398 is 2.7% (in M390 steel 1.9%). This provides higher steel hardness. Due to this, with proper heat treatment of M398 steel, its hardness reaches 64 HRC. A large amount of chromium in its composition - 20% affects corrosion resistance. The composition also includes other alloying elements, such as: vanadium - 7.2%, molybdenum 1%, silicon 0.5%, tungsten 0.7% and manganese 0.5%. Therefore, due to the unique production technology and verified composition, M390 steel has a number of advantages over other steels.

The increased chromium content provides the steel with good corrosion resistance, and the increased vanadium content provides very high wear resistance. The carbon content has increased by 30% compared to its predecessor, the M390 steel, which gave it a hardness of 63 - 64 (according to some sources 65) units on the Rockwell scale. As of 2024, M398 stainless steel is considered one of the best powder stainless steel grades, which is confirmed not only by its characteristics, but also by its price.

Application

Due to its property profile, BÖHLER M398 MICROCLEAN can be used for the following applications:

Check valves;
Screws for casting machines;
Inserts for injection molding;
Components with high wear resistance;
Cutting tools and knives.

Heat treatment recommendations

The steel delivery condition is soft annealed with a maximum hardness of 330 HB.

Hardening

Austenitization temperature: from 1120 °C to 1180 °C;
Holding time after through heating:
- 20 - 30 minutes for hardening temperature from 1120 °C to 1150 °C;
- 5 - 10 minutes for a hardening temperature of 1180 °C;
Hardening medium: oil, N₂.

Achievable hardness

From 60 to 63 HRc

Hardened for maximum corrosion resistance

Deep freezing to transform residual austenite;
Slow heating to tempering temperature;
Time in the oven is 1 hour for every 20 mm of workpiece thickness, but not less than 2 hours;
Tempering: from 200 °C to 300 °C.

Hardened for maximum wear resistance

Deep freezing is recommended;
Deep freeze treatment immediately after quenching results in increased tempering hardness values at austenitizing temperatures ≥ 1150 °C, (Risk of stress cracking);
Slow heating to tempering temperature;
Time in oven 1 hour for every 20 mm (0.79 in) of workpiece thickness, but not less than 2 hours;
To achieve complete transformation of the residual austenite, a triple tempering at 20 °C above the secondary hardening maximum is required.

Table: heat treatment of stainless steel M 398
Heat treatment of steel M398
Stress relieving
Temperature	max. 650 °C	Soft annealed material: for stress relief annealing after mechanical processing, hold the material at temperature in a neutral atmosphere for 1-2 hours after complete heating, then slowly cool the furnace at 20 °C [68 °F]/hour to 200°C [392 °F], then cool in air.
Temperature		Hardened and tempered material: the temperature for stress relief annealing should be approx. 50 °C [122 °F] below the previously selected tempering temperature. Other procedure as for stress relief annealing of soft annealed material.
Hardening and Tempering
Temperature	1,120 to 1,150 °C	For hardening, hold the material at the specified temperature for 20-30 minutes after complete heating and quench quickly. Cool the material to approx. 30 °C [86 °F]. Immediately afterwards, the material can be deep-frozen for 2 hours (at -80 °C [-112 °F]) for residual austenite transformation. Tempering should also be carried out immediately.
Temperature	1,151 to 1,180 °C	For hardening, hold the material at the specified temperature for 5-10 minutes after complete heating and quench quickly. Cool the material to approx. 30 °C [86 °F]. Immediately afterwards, the material can be deep-frozen for 2 hours (at -80 °C [-112 °F]) for residual austenite transformation. Tempering should also be carried out immediately.
Temperature	200 to 300 °C	Tempering treatment: for maximum corrosion resistance, heat the material slowly and temper once for 1 hour/20 mm material thickness, but for at least 2 hours. Take slow heating into account and cool the material to approx. 30 °C [86 °F] after each heat treatment step.
Temperature	540 to 560 °C	Tempering treatment: for maximum wear resistance (without sub-zero cooling), temper the material 3 times for 1 hour/20 mm material thickness, but at least 2 hours. Allow for slow heating and cool the material to approx. 30 °C [86 °F] after each heat treatment step.
Temperature	510 to 530 °C	Tempering treatment: for maximum wear resistance (with sub-zero cooling), temper the material 3 times for 1 hour/20 mm material thickness, but at least 2 hours. Allow for slow heating and cool the material to approx. 30 °C [86 °F] after each heat treatment step.

Physical properties (20 °C)

Modulus of elasticity at 20 °C - 231 x10³N/mm²
Density at 20 °C - 7.46 kg/dm³
Specific heat capacity at 20 °C - 490 J/(kg.K)
Thermal conductivity at 20 °C - 15.2 W/(m.K)

Thermal expansions between 20 °C | 68 °F and ...

Table: thermal expansion of steel grade M398
Temperature °C	100 °C	200 °C	300 °C	400 °C	500 °C
Thermal expansion 10^-6 m/(m.K)	10,4	10,6	10,9	11,2	11,5

Mechanical processing

Turning with sintered carbide

Table: turning of M398 steel with sintered carbide
Cutting depth mm	0,5 - 2	1 - 4	4 - 8	more than 8
Feed mm/rev.	0,1 - 0,3	0,2 - 0,4	0,3 - 0,8	0,5 - 1,5
Cutting speed v_c (m/min)	130 - 200	100 - 170	70 - 120	30 - 70
BÖHLER class	LCP15T, BCM25T	LCP15T, LCP25T, BCM25T	LCP25T, LC240F, BCM40T	LC240F
ISO class	P15, M25	P15, P20, M25	P20, P30, M40	P30, P40

Condition: soft annealed. The figures given are approximate.

High speed steel turning

Table: turning of M398 high speed steel
Cutting depth mm	0,5	3	6
Feed mm/rev.	0,1	0,4	0,8
BÖHLER / DIN class	S700 / DIN S10-4-3-10
Cutting speed v_c (m/min) Tool stability 60 min.	30 - 20	20 - 15	18 - 10
Rake angle	14°	14°	14°
Clearance angle	8°	8°	8°
Tilt angle	-4°	-4°	-4°

Milling with sintered carbide

Table: milling of M398 steel with sintered carbide
Cutting speed v_c (m/min)	150 - 180	130 - 160	80 - 140
BÖHLER class	BCH10M, BCM35M	BCH30M, BCM40M	BCM40M, BCP40M
ISO class	H10, M35	H30, M40	M40, P40
F_z Milling 90° mm	0,1 - 0,25	0,1 - 0,25	0,1 - 0,3
F_z Milling 45° mm	0,15 - 0,6	0,15 - 0,6	0,15 - 0,6
F_z High cutting feed mm	0,6 - 1,8	0,6 - 2,0	0,6 - 2,0

Drilling with sintered carbide

Table: drilling M398 steel with sintered carbide
Drill diameter mm	3 - 8	8 - 20	20 - 40
Feed mm/rev.	0,02 - 0,05	0,05 - 0,12	0,12 - 0,18
BÖHLER / ISO клас	HB10 / K10
Cutting speed v_c (m/min)	50 - 35	50 - 35	50 - 35
Sharpening angle	115° - 120°	115° - 120°	115° - 120°
Gap angle	5°	5°	5°

Condition: soft annealed. The figures given are approximate.

Advantages and disadvantages

Advantages of stainless steel M398:

High corrosion resistance: this steel has good corrosion resistance, which makes it ideal for use in some aggressive environments.
Good machinability: M398 is quite easy to weld, cut and grind, which simplifies the process of manufacturing parts and structures.
Strength and hardness: the steel has high strength and hardness, which allows it to withstand significant loads and wear.
Resistance to high temperatures: M398 can withstand fairly high temperatures without losing its properties, which makes it suitable for use in high-temperature conditions.
Aesthetic appearance: mirror, polished, matte, ground surface treatment of the steel gives it an attractive appearance, which allows it to be used for decorative purposes.

Disadvantages of stainless steel M398:

Higher cost: compared to other steel types, M398 is more expensive due to its special properties.
Difficulty in machining: although the steel is easy to machine, some types of machining may require specialized equipment and skills.
Sensitivity to chloride environments: despite its high corrosion resistance, M398 may be susceptible to corrosion in environments with high chloride content.
Limited ductility: the steel has limited ductility, which may make it difficult to use in parts requiring complex geometries.

Conclusion:

M398 stainless steel is a versatile material that finds wide application due to its outstanding properties. However, before use, both advantages and disadvantages must be considered to ensure the best choice for a particular application.