Effect of Heat Treatments and Surface Melting by TIG on the Erosion of a Grey Cast Iron

Ahmed El Gehani

عضو جديد
إنضم
25 يناير 2017
المشاركات
12
مجموع الإعجابات
1
النقاط
3
Introduction
Grey cast iron became important engineering materials due to their high strength and toughness and relatively low price in addition to their excellent castability and machinability. However, under severe service conditions their performance and reliability can be limited by erosion.
In recent years, high energy beams such TIG has been used to improve erosion of cast irons. In this technique, the surface is partially re-melting and rapidly solidifies. The rapid cooling during solidification leads to the formation of large amount of cementite instead of the graphite and the sliding erosion resistance were improved significantly due to the hard surface developed. The present work aims to use the TIG process as a heating source to modify the surface by local re-melting and hence improve hardness and erosion resistance and emphasizing on the modified structure from one side and to the erosion .
resistance at different impinging angles from the other side.

Methodology
The chemical composition is: 3 wt.% C, 2 wt.% Si and the remainder is iron. Samples of 20mm long, 20mm wide and 12.5mm thick were surface ground mechanically, polished and etched with 3% nital. Different types of heat treatments were performed including annealing, normalizing, water quenching, and oil quenching. Some specimens were melted by electric arc generated between the tungsten electrode and the specimen using currents ranged between 100 and 200A and the voltage was set at 72 V. During TIG melting, a continuous flow of argon was applied onto the surface to protect the melted region from oxidation and undesirable contamination. These cross sectioned samples were analyzed for microstructure and micro hardness by Rockwell method. All samples were eroded in a typical sand blasted erosion apparatus at angles of 30° and 90°, with constant distance between the nozzle and sample.

Results and discussions


  1. [*=left]Effect of heat treatment:
The graphite decomposition process after ten hour austenizing time at 950°C, as shown in the figures also martensite can be seen near the flakes with ferrite matrix.


  1. [*=left]Effect of surface melting:
During re-melting process the surface was locally melted and rapidly solidified by self-quenching. Graphite flakes were completely dissolved and the structure becomes dendritic with no sign of graphite. When the sample moved away from arc, the melted zones solidify quickly. The structure consist of transformed austenite (appear dark) and cementite (appear white). This fusion zone solidifies as austenite dendrite; on cooling below the eutectic temperature, the last liquid solidifies as eutectic of austenite and cementite. On further cooling to room temperature, the austenite phase will transform to pearlite or to martensite if the cooling rate during solidification is very high.

Conclusion
Among the various heat treatments applied, quenching in water produced the highest hardness and showed significant reduction of erosion rate and the improvement in erosion resistance after remelting was attributed to the high hardness. The erosion rate at angle 30° is maximum while it is minimum at 90°.
The micro hardness of the treated layers is increased. Increasing the current affected the scale of the microstructure and increased the hardening depth and the heat affected zone depth. However, the hardness level of the sample processed at 100 A current is higher than those treated at 150 and 200 A.
The experimental results of erosion test showed a significant improvement in erosion resistance after melting treatment.​
 

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