Optimization of Surfacing Welding of Forged Steel Valve Bodies (Part Two)

Optimization of Surfacing Welding of Forged Steel Valve Bodies (Part Two)

Apr 02, 2021


2.2 The results of penetrant inspection
A gas hole is found on the surfacing surfaces of samples 1, 2, 3 and 5 of forged steel valves, and there are many slag inclusions for the sample 2. The sample 3 has a small amount of slag inclusions. There are gas holes on the surfacing surface for the sample 4. Samples between 6 and 10 as well as 14 have defects on their surfacing surfaces. Cracks are found on the surfacing surfaces of the samples 11, 12, 13 and 15. A lot of slag inclusions were found on the surfacing surface of the sample 16.
 
According to the Table 1, it can be seen that the two parameters of voltage and surfacing surfaces' heights have little effect on defects of surfacing surfaces. When the current is 100A, the surfacing surface's defects are mainly gas holes and slag inclusions. When the current is 150A, there is basically no defect. When the current is 200A, the defects are mainly cracks.
 
2.3 Results of hardness inspection
Table 2 Hardness inspection records
Items Hardness (1) Hardness (2) Hardness (3) Average values Items Hardness (1) Hardness (2) Hardness (3) Average values
1 32 31 32 31.7 9 32 33 34 33
2 34 37 30 33.7 10 32 32 32 32
3 39 37 38 38 11 32 30 34 32
4 35 35 35 35 12 30 30 26 28.7
5 36 34 35 35 13 36 36 35 35.7
6 31 30 31 30.7 14 23 25 24 24
7 32 32 33 32.3 15 33 35 37 35
8 35 35 37 35.7 16 35 31 28 31.3
 
(1) The influence of current on hardness
The height of the surfacing welding surface is 3mm. When the current changes between 100A, 150A and 200A, the hardness decreases with the increase of the current; when the height of the surfacing welding surface is 6mm, the hardness shows an overall downward trend as the current increases. However, the changing trend is relatively slow. When the height of the surfacing welding surface is 11mm, the current changes almost have no effect on hardness.
 
(2) The effect of surfacing surfaces' heights on hardness
When the current is constant at 100A, the hardness of the sample 3 reaches the highest value of 38 HRC. When the current is constant at 150A, the sample 8 reaches the highest hardness value of 35.7 HRC. When the current is constant at 200A, the sample 14 reaches the minimum value of 24 HRC. When the current is between 100A and 150A, the hardness changes little with the height of the surfacing welding’s surfaces. When the current is 200A, the height of the surfacing welding's surfaces has a great influence on the hardness.
 
2.4 Metallographic test results
The samples 1, 5, 78, 9 and 12 were subjected to metallographic analyses. A small amount of dendritic Co-Cr-W solid solution (A) primary crystal plus solid solution and Cr-W composite carbide eutecticum (Hereinafter this text will be abbreviated as primary crystal plus eutecticum.); the sample 5 has many primary and eutectic crystals, and the grains are coarse; there are a large number of primary crystals plus eutectic crystals for the samples between 7 and 9; a very small amount of primary and eutectic crystals are found for the sample 12, and the crystal grains are relatively coarse with layered precipitates.
 
The main factor for the different hardness of samples 1 and 5 is the difference in matrix structure content. Grain coarsening occurs for the sample 5. The sample 7, 8 and 9 have good results in terms of surface forming of surfacing welding, penetration and hardness inspection. The sample 12 has very little matrix phase content, serious grain coarsening and layered precipitates.
 
3. Conclusion
Through the comprehensive analysis of the above testing results, it was found that the influence rules of current, voltage and heights of the surfacing welding's surfaces on the formation, hardness and metallographic structure of the surfacing welding's surfaces are concluded as follows:

When the current is 100A, the molding is not good due to the small current. When the current is 200A, which is too great, the molding condition has improved, but there are defects such as undercuts. When the current is 150A, the forming condition is good due to the suitable current.
 
When the voltage is 15V or 25V, the forming of the surfacing surface is ideal. When the voltage reaches 35V, few pointed shapes appear on the forming of the surfacing surface.
 
At a current of 100A, the existence of the molten pool is short and the heat input for the base metal is insufficient due to the low temperature, resulting in main defects such as gas holes and slag inclusions; at a current of 200A, stress concentration is the main cause of cracks; when the current is 150A, no defects are found.
 
The dilution rate is the main reason that affects the hardness of the surfacing surface. When the current is 100A or 150A, the impact is small; the hardness changes little, and the overall hardness is at a higher value. When the current is 200A, the dilution rate is high because of the high current, and the hardness is small; as the current increase, the overall hardness shows a parabolic downward trend.
 
When the surfacing surface is 3mm, the dilution rate is high and the hardness is small. When the height of the surfacing surface is greater than 6mm, the hardness is basically not affected due to the low dilution rate, and the overall value is high. The hardness increases with the increase in the height of the welding surface.
 
As the current increases, the matrix phase content decreases. If the surfacing surface is too high, the structure of the surfacing surface will be embrittled. Higher content of dendritic Co-Cr-w solid solution (A) primary crystal plus solid solution and the eutectic phase of Cr-w composite carbide in the surfacing surface is the most needed organization for the surfacing surface.

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