AISI 420 FM - Tool Steel - 1.2085 - X33CrS16 - Mold steel

Where can tool steel 420 FM be used?

Applications for this corrosion resistant tool steel include: Blow molds, packaging, electronic industry, measuring gauges, and optical instruments. Due to the better corrosion resistance, the maintenance requirement for molds is reduced. The 420 FM can be used for machine parts or medical devices which are subjected to wear or require regular cleaning or maintenance due to its hardness and wear resistance. 

Is tool steel 420 FM a stainless steel?

To be classed as a stainless steel the steel has to have a mass fraction of at least 10.5% of chromium. With an alloy content of 15 - 17% chromium the 420 FM can be classed as stainless steel.

Is AISI 420 FM tool steel corrosion resistant?

With a mass fraction of 15 - 17% of chromium the 420 FM is a corrosion resistant steel. 

420 FM tool steel general corrosion

In general this steel grade shows a good corrosion resistance in acidic environments, regular maintenance and cleaning can improve the lifespan of this material grade. Treating or polishing the surface to a high finish can prevent the possibility for rust to attach itself and spread further. Though not usual, heat treating or welding this material grade can affect the corrosion resistance. 

Is tool steel 420 FM magnetizable?

Yes, 420 FM tool steel, as a martensitic steel, has the capability to be magnetized and is suitable for magnetic clamping.

Heat treatment for tool steel 420 FM

Generally, as this steel grade is delivered in the pre-hardened condition, further heat treatment is not required. The following procedures can be applied if a higher hardness is necessary.

Annealing AISI 420 FM

Heat the 420 FM work pieces uniformly to 1562 - 1616°F (850 - 880°C) and hold them for 2 to 5 hours, then slowly cool in the furnace to a temperature of 932°F (500°C), after that the material can be cooled down further in air, ash or insulating materials.

Stress relieving tool steel 420 FM

To eliminate surface and/or residual stresses from machining, heat the work piece to 842°F (450°C), then cool down slowly in the furnace. At a higher temperature this steel grade tends to scale.

Hardening tool steel 420 FM

Heat 420 FM stainless steel uniformly through to 1832 - 1886°F (1000 - 1030°C) and hold it at this temperature for 30 minutes. Then quench the work pieces in oil.

Quenching AISI 420 FM

Quenching this material will transform austenite into martensite, a hard and brittle phase.

• Oil
• Compressed gas (N2)

Tempering tool steel 420 FM

To reduce the brittleness of the phase change, after quenching, but retain the hardness achieved, heat the 420 FM slowly to a tempering temperature of 392 - 572°F (200 - 300°C), hold for 2 hours per 25/32 inch (20 mm) thickness. Double tempering is recommended. Slowly cool in the furnace.

Dimensional changes of the 420 FM tool steel

Though not normally done, extensive heat treatment of the 420 FM can lead to warping or dimensional changes.

Sub-Zero treatment the 420 FM tool steel

After the quenching process some austenites can be retained. These retained austenites can be changed to martensite by sub-zero treatment. After treating the steel at these low temperatures it is necessary to temper the material to give it a good property balance again.

Sub-zero treatment can positively affect the wear resistance, dimensional stability, hardness and strength of the 420 FM. 

Electrical Discharge Machining (EDM) of the 420 FM

As a pre-hardened material the 420 FM can be machined by EDM. EDM makes it possible to remove intricate and precise shapes which might be harder with conventional machining. Polishing or heat treatment might be needed after EDM to relieve introduced stresses or to restore surface characteristics.

Surface treatment of the 420 FM tool steel

Any surface treatment has its own advantages and should be chosen with needed properties, required finish and/or what environment it is used in in mind. Here are a few examples of surface treatments for the 420 FM:

Nitriding the 420 FM tool steel

Nitriding introduces nitrogen into the surface of the material which can enhance hardness and wear resistance of the 420 FM.

Passivation of the 420 FM tool steel

Passivation is using acids to remove free iron from the surface of the material it is used on which in turn enhances the protective oxide layer to improve the already existing corrosion resistance.

Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD)

Both of these processes add a thin layer of protection onto the surface of the material, this can enhance hardness, reduce friction and help to enhance wear resistance. 

Plating the 420 FM tool steel

Plating, like hard chrome plating or electroless nickel plating can enhance wear resistance, corrosion resistance and can also be used to enhance the material for decorative purposes.

Black oxide coating (bluing) tool steel 420 FM

Black oxide coating can enhance the corrosion resistance of the parts but is mostly done for esthetic purposes. It will give parts a blue-black coloring that will reduce light reflecting off the surface.

Machinability for the 420 FM tool steel

With the addition of Sulphur this steel grade has a good machinability. As a pre-hardened material the 420 FM can be machined by milling, drilling and turning. To prevent tool wear, cutting tools should be well maintained and kept sharp. Cutting fluids should be used to reduce friction and wear of the tool and to reduce heat created between the tool and workpiece.

After machining the 420 FM, the workpieces may need to be stress relieved, though this is less of a concern for the pre-hardened condition but can still affect and be important for continued dimensional stability of precision components. 

Forging tool steel 420 FM

Uniformly heat the parts to a temperature range of 1922 - 1562°F (1050 - 850°C), when finished forging cool the workpieces slowly down.

Wear resistance of tool steel 420 FM

The 420 FM has a wear resistance of 3 on a scale where 1 is low and 6 is high.

Tensile strength of the 420 FM tool steel

The 420 FM has a tensile strength of approx. 163.1 KSI on delivery (0.145 KSI = 1MPa). In order to reach this value, a tensile test is performed to show how much force is needed to stretch or elongate a sample before it breaks.

Working hardness for tool steel 420 FM

The working hardness for the 420 FM tool steel is approx. 308 (on delivery) - 456 BHN (33 (on delivery) - 48 HRC). 

Specific heat capacity for the 420 FM tool steel

The specific heat capacity of the 420 FM tool steel at room temperature is at 0.46 J/g-°C (0.109 BTU/lb-°F). This value shows how much heat is needed  to heat 1lb of material by 1 Fahrenheit.

Continuous TTT-diagram for tool steel 420 FM

This diagram shows micro changes over time at different temperatures. Those are important during heat treatment as they show information about the optimum conditions for processes such as hardening, annealing and normalizing.

420 FM Steel Properties

Hot working tool steel 420 FM

Hot work should be done in recommended temperature ranges to avoid damages and overheating. Stress relieving might be necessary to eliminate internal stresses.

Cold working tool steel 420 FM

Due to the pre-hardened condition, cold working this material can be challenging and might introduce stresses that can lead to cracking, increased tool wear and greater forces might be needed to achieve the desired deformation.

Steel Density of the 420 FM tool steel

Typically the density of 420 FM tool steel is 0.28 lb/in3 (7.72g/cm3) at room temperature.

Thermal conductivity of tool steel 420 FM

The heat conductivity for tool steel 420 FM is at 23.9 W/(m*K) (166 BTU/(h-ft*°F)) at room temperature.

Thermal expansion coefficient of tool steel 420 FM

The following table shows expansion or contraction at various temperatures, which may be very important for high temperature works or when working with high temperature changes.

Modulus of elasticity (Young’s Modulus) of tool steel 420 FM

The stress and strain modulus or modulus of elasticity (Young’s modulus) for P20 tool steel  is at 31.183 KSI (215 GPa).

Is the 420 FM a knife steel?

The 420 FM is not typically classified as knife steel. A good knife steel has a good balance of corrosion resistance, edge retention, chipping resistance and is easy to sharpen. Though the 420 FM has some of those properties others are too low, like the edge retention for example, to make a good quality knife. Most knives require approx. 56 - 60 HRC in order to maintain a good edge and have the required toughness.

Conclusion AISI 420 FM

The 420 FM is a great choice for plastic molds offering a great combination of corrosion resistance, wear resistance, dimensional stability as well as machinability. It has a great balance of toughness and hardness and is able to withstand mechanical stresses.

Datasheet for tool steel 420 FM

As a steel supplier we provide you with the datasheet for tool steel 420 FM in PDF format.

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