3.0 Detail of Processes

3.1 Extrusion

First, hot direct extrusion process is performed on the raw material to get the required diameter for valve.

3.2 Cutting

The rod is then precisely cut to required length for upsetting process.

3.3 Friction Welding

Friction welding is a process of joining valve head with the straight rod using lathe machine. The straight rod rotating at high speed comes into contact with the valve head. Heat produced from friction between the two surfaces causes the surfaces to melt and creates high-integrity joint between the two parts. This process is used to join valve head to the valve stem in production of engine valve especially exhaust valve in which the valve head has to withstand higher temperature and pressure than the inlet valve. This process will ensure that the joint formed between the valve stem and valve head is smooth and continuous.

3.4 Upsetting

The bar then undergoes electrical upsetting for a smooth material flow and reduced fatigue during forging process without changing the material composition.The bar is clamped in the upsetting machine and heated to about 1050°C by electrical resistance between two contacts. As the bar reaches 1050°C, more material is forced throught the contacts by a hydraulic ram until pre-form is formed. Molten bulb is formed at the top end of the bar because of the heat produced due to the electrical resistance. The pre-form will then enter the forging stage.

3.5 Forging

Forging is the process in which metal, either cold or heated, is shaped into a desired geometry using localized compressive forces from the multiple blows with a drop hammer or pressure from hydraulic press. After the upsetting process, while the bulb of the bar is still in molten form, it undergoes forging. After forging, the valve is examined for any unwanted deformation that may form during forging.

3.6 Heat Treatment

After forging, the valve will undergo heat treatment because the stainless steel is hard and brittle with high severe internal stresses for the subsequent processes. Heat treatment or tempering rearrange the microstructure of the material. It will relieve the internal stresses, improve machinability and increase toughness of the stainless steel. The valve is heated to a temperature of about 700° C to 900° C. It is then quenched to retain the high temperature phase at room temperature. The resultant strength, hardness, and ductility depend on temperature to which the steel is heated during the tempering process.

3.7 Stellite Welding

Stellite is a special alloy that is placed into the valve seat. This is to improve the corrosion resistance and high temperature wear resistance of the valve especially exhaust valve for long service life. High residual stresses in the valve are relieved and hardness of the valve are improved through stellite welding process. Gas Tungsten Arc Welding (GTAW) or TIG welding is used in this process. It provides high quality weld and is always chosen as the welding process when precise and high quality welding is needed. During the welding, arc is formed between the metal being welded and non-consumable tungsten electrode. To shield the electrode and molten weld pool, gas is fed through the torch. Filler wire is added to the weld pool separately.

3.8 Turning Process

In this process, the sample is turned to reach the required diameter for the head part of the exhaust valve. After this, another turning process will be carried out to make a groove at the end of the stem. Besides the head and the end of stem of the valve, turning operation also will be performed on the neck region of the valve in order to make a curve at this region. Excess materials from these parts are removed by using this process.

3.9 Seat Grinding

To ensure the valve seat seals the combustion chamber perfectly during the compression and power strokes, the valve seat need to be uniformly flat. Grinding process can be carried out to produce a smooth finish on flat surface of valve seat. The seat angle can be 20, 30 or 45. During grinding, cutting oil is sprayed over the contact surface to prevent deformation. Accuracy is important in this process to make sure the complete sealing of combustion chamber and the seat insert of exhaust valve.

3.10 Surface Treatment

In order to improve the surface properties, a layer of hard chromium or nitride or other surface treatment is done as per the requirement and application. Treatment of the surface can increase in hardness of the surface and greater wear resistance, coupled with an increase in fatigue strength on account of the residual compressive stress associated with this surface treatment process.

3.11 Final Inspection

Quality inspection is then carry out in laboratory by qualified engineers to measure the overall performance of the exhaust valve. The tolerance parameter, the number of valves passed, the number of rejections will be checked in this process. This can ensure all the exhaust valves produced meet the standards.

3.12 Packaging

100% inspection builds confidence amongst packing staff to pack quality at valve in special packing material.