Technical Information: Surface Treatments

Various treatments are available to enhance the life and performance of your valves, these include the following:

Pulsed Plasma Nitride

Valves for internal combustion engines must have high resistance to thermal stress caused by frequent temperature changes. They must remain ductile but acquire good anti-friction properties to reduce wear in the area of the valve guide. The valve seat must be able to resist the corrosive effects of hot exhaust gases from the engine.

Pulsed Plasma Nitriding (Furnace supplied by Eltropuls) improves the resistance to wear and the anti-friction qualities by increasing the surface hardness along the valve stem, while retaining the inherent corrosion resistance of the material at the valve seat. Pulsed Plasma Nitriding allows high temperature metallurgical reactions to occur at low work surface temperatures, plasma is produced by applying high voltage through a low pressure gas (a mixture of hydrogen and nitrogen) causing it to ionise, using this high energy (but thermally low temperature), plasma will diffuse nitrogen into the surface of the valve.

Some of the advantages of Pulsed Plasma Nitriding are:

1. This process is applied at low temperature so it does not affect the original core properties of the substrate material.

2. The nitride layer is more uniform and there is less deviation from mean values, when compared to salt bath or gas nitriding.

3. The anti-friction qualities between the valve stem and the valve guide have been substantially improved.

4. Plasma Nitrided surfaces offer better protection against adhesive and abrasive wear

5. Due to the low temperature process the dimensional stability of the valve is improved.

6. Valves can be masked to allow specific parts to be treated

7. Using this process it is possible to allow other treatments to be applied to further enhance the properties of the valve

8.By using this process it is also possible to produce a case structure without an Iron Nitride compound (White) layer and this allows the deposition of a subsequent surface coating onto the nitrided surface, to produce a multi layer coating.

eg: TiAlCrYN (PVD Coating) and DLC - (PACVD Coating)

The basic procedure for Pulsed Plasma Nitriding valves are as follows:

The valves are washed to remove any traces of oil and machining residues, they are then placed inside the vacuum chamber in a manner that will permit the plasma to gain access to all of its important surfaces. The furnace is then closed, the atmosphere in the chamber is evacuated to give a vacuum. A number of purges and evacuations of the furnace atmosphere are made to ensure that there is no residual air inside the chamber. A voltage is then applied with a controlled gas mixture to produce a plasma. This plasma is first used to sputter cleaned and remove any passive layers on the surface.

During these stages the furnace load is heated by both the furnace wall heaters and the plasma until the nitriding temperature is reach at which stage the load is held for a specific amount of time to allow the plasma to produce required nitride. At the end of the nitriding cycle the valves are then cooled down in a vacuum giving a slight grey appearance on the nitrided areas of the valve.

Example of Process Procedure:

1. Initial heating

2. Sputter Cleaning

3. Nitriding

4. Cool down cycle

The table below shows the case depth and surface hardness achieved on different valve steels from pulsed plasma.

 

Trufftride

(AB1 or TF1)- (the process used depends apon the specification of the valve).

Gives a hard layer of between 72 to 74 Rockwell 'C' over the complete valve of approx. 10 - 20 microns in depth, and gives excellent wear properties in a cast iron or bronze guide with the added benefit of stress relieving the valve. This type of treatment gives a black mottled finish all over the valve.

Tufftride Process

TF1 AB1
1. De-grease 1. De-grease
2. Process in air circulated furnace@350/250 °C. 2. Process in air circulated furnace@350/250 °C
3. Process in (TF1) for 40 Min. @ 600 °C. 3. Process in (TF1) for 40 Min. @ 600 °C.
4. Quench in oil 4. Process in salt bath (AB1) @380 °C.

5. Wash components in hot water then warm

5. Quench in cold water
6. Oil components 6. Wash components in hot water then warm

Hardness achieved on different valve steels from pulsed plasma

Material

Minimum Surface
Hardness
(@50 Gram load)

Minimum Case
Depth mm

214N (349S52)

1000 HV0.05

5 mm

1.4882

1000 HV0.05

5 mm

Nimonic 80A

700 HV0.05

5 mm

EN52 (401S45)

1000 HV0.05

10 mm

EN24 (817M40)

600 HV0.05

10 mm

Hard Chrome Plating

Gives the stem added durability by depositing chrome on the stem to guide area of the valve of approx. 32 - 7 2 microns in thickness, this gives good compatibility if the valve is made in 214N (Stainless) and is to be used in a cast iron guides. This type of treatment is only on the valve stem.

(Maximum thickness of chrome is 0.3 - 0.4mm (0.012" to 0.015") deposits in excess go brittle.)

Cobolt based Seat Deposit

A cobolt based deposit No.6 is placed on the exhaust valve seat face which enhances the seat hardness (Rockwell'C' of between 38 to 42 HRc) which enable it to be used with unleaded fuel or high stressed engines e.g. turbos or superchargers or engines that are generally hard on valve seats.

Sodium Filled Valves (Not Available)

Surface Treatment Note: 25 micron's = 0.001

Surface Treatment

Hardness Rockwell 'C'

Hardness Vicker's '30'

Thickness 0.000*" (Tenth's)

Thickness microns (mm)

Hard Chrome Flash

52-54 HRc

544 - 577 Vicker's

12 - 3 tenth's (0.00015 - 0.0003)

32 - 7 2 mm's

Tufftride AB1

70-74 HRc

 

4 - 8 tenth's (0.0004 - 0.0008)

10 - 20 mm's

Tufftride TF1

70-74 HRc

 

4 - 8 tenth's (0.0004 - 0.0008)

10 - 20 mm's

Plasma Nitriding

See Above Table

See Above Table

See Above Table

See Above Table