Currently, large high pressure steam turbine main steam valve body, cylinder and other pieces of equipment, materials, mostly steel, due to thick-walled steel castings, casting performance is poor, often within the sandbags, shrinkage, etc., and thermal stress at work under stress, the defect area will have a more serious crack defects, a direct impact on equipment life, and seriously affect the safe operation of the unit. To this end, research and analysis of steel casting material properties, a reasonable choice of Welding Electrode Manufacturer materials and welding steel casting process has become a necessary condition for welding defects.
A steam turbine cast steel material properties and structural characteristics
1.1 turbine cast steel material properties
Turbine castings generally work at high temperature and pressure. Selection of material for pearlitic heat-resistant steel, the steel used has ZG15Cr1Mo1V and so on. ZG15Cr1Mo1V is a good overall performance of hot strong pearlite steel, long-term work at 570 â„ƒ. However, the poor performance of steel casting, easy to crack, and the cooling rate of the heat treatment of steel is also very sensitive, easily caused in the steel casting and properties of non-uniform. Table 1 ZG15Cr1Mo1V chemical composition, Table 2 ZG15Cr1Mo1V mechanical properties.
1.2 Structural Features
Steam turbine cast steel cylinder main body, high and medium pressure main steam valve body, etc., are closed or semi-enclosed rotary shell, shell diameter, cross-sectional area, shape changes are large, cast steel wall thick and the wall thickness is also large.
2 welding weld chemical composition calculation
2.1 the transition coefficient
Electrode melts into the weld metal in the alloy depends on the number of electrodes and welding slag deposited metal interactions. With the welding conditions, the alloy element oxidation, evaporation, splashing and other losses are different, that each of alloying elements in the welding seam in the transition to go to different coefficients. Alloying elements during welding the transition coefficient μ:
μ = (C fill / C electrode) × 100% (1)
Where C is filled - filled metal in an alloy element in the content; C electrode - an electrode in the alloy elements in the content.
In addition, the base metal and weld filler metal composed by a certain percentage, weld in a content of alloying elements by equation (2) calculation:
Where C is the base material - an alloying element in the content of the base metal; θ - the share of the base metal in the weld volume ratio.
Set and base metal fused together, said bottom seam weld, the weld than θ1 = 0.3; To facilitate estimation, the other pile layer deposited weld metal fusion than θ2 = 0. Assuming no phase with the base material from the fusion of the weld surfacing layer of base metal dilution effect, this assumption is easy to calculate the number. Under normal circumstances of deposited metal layer 3, its fusion than θ3 under 0.05. Therefore, the assumption of the analysis and discussion of the problem will not have a significant impact.
When the weld is titanium calcium type coating, the various alloying elements of the transition coefficient values ??are as follows :
2.2 calculate the chemical composition of different welding seam
ZG15Cr1Mo1V ENiCrFe-3 steel using austenitic stainless steel electrodes and electrode E2-26-21-16 (A402), E0-19-10Nb-16 (A132), respectively, after Welding Alloys the bottom seam welds and other weld overlay chemical composition results in Table 3 to Table 5.
2.3 Analysis of different welding weld
2.3.1 with ENiCrFe-3 electrode (nickel) steel welding ZG15Cr1Mo1V
The resulting weld metal in the bottom seam welding with Ni ≥ 39.65%, Ni Fe ≥ 71.59%, are iron-based alloy; other cladding layer containing Ni ≥ 56.64%, with Fe less, only 8.5% are Nickel based alloys. All welding seam (including the bottom and surfacing) are austenite phase of Fe-based alloys and nickel-based alloy.
Welding seam of the austenite phase of Fe-based alloys and nickel-based alloys have great advantages, mainly in the following areas.
a. the composition of nickel-based weld allows a large range of different specifications to reduce the impact due to welding seam dilution rate, and thus the impact on the composition of the weld. b. nickel-based weld is not sensitive to hydrogen, hydrogen solubility, the hydrogen of the base material to overcome the harmful effect is very obvious. c. nickel-based weld thermal expansion coefficient closer to the base material, which is running in the process of welding and residual stresses due to thermal stress are much smaller and attached. d. Nickel-base weld metal in the base metal can inhibit the proliferation of carbon, reducing the carbon diffusion because the adverse consequences.
2.3.2 electrode welding ZG15Cr1Mo1V with A402
From Schaeffler (Schaeffler) organization chart shows, when the bottom of the weld seam, and other surfacing are a lot of reserves austenite single-phase austenite.
However, single-phase austenite in the welding seam and running, there are some problems that need solutions.
a. As the austenitic weld metal than ZG15Cr1Mo1V high linear expansion coefficient of about 1.4 to 1.6 times in the welding thermal cycle under the effect of weld expansion, contraction of steel castings by the degree of binding limits, certainly in the welding seam and base metal junction cause greater stress state. Running, start and stop due to temperature change and thermal cycling role in the joint interface will generate additional heat stress, with the increase in the number of such thermal cycling, thermal stress will cause the joint thermal fatigue damage. b. between austenitic welds and base metal carbon proliferation, resulting in the seam zone near the base metal side there decarburization. c. precipitation of σ phase austenitic weld metal embrittlement problems in the formation of austenite grain boundary precipitates, the austenitic weld toughness decreased.
2.3.3 electrode welding ZG15Cr1Mo1V with A132
Weld metal in the bottom of the proceeds of the welding seam composition of chromium, nickel, equivalent to:
Organization from the Schaeffler diagram shows the organization of the underlying weld metal is martensite plus austenite; and other metal surfacing layer is austenite plus martensite tissue, may also occur ferrite, the organization is reluctant to get the welding seam, it is generally not used.
2.4Ni-Fe-Cr ternary alloy state diagram analysis
In addition, also available from the Ni-Fe-Cr ternary alloy state diagram analysis, the same conclusion. Figure 2 is 650 â„ƒ when the Ni-Fe-Cr ternary alloy state diagram of the organization. The figure shows, many of austenitic steel welding material very close to α γ two-phase and σ phase precipitation in the region; and nickel-based welding consumables requires a lot of Fe diluted in order to make the weld chemical composition of σ-phase precipitation into the area.
a. ZG15Cr1Mo1V welding, you can not use chromium, low nickel austenitic heat-resistant 18-8 stainless steel electrode, welding metal martensite will occur.
b. selection of chromium, nickel, higher levels of heat-resistant austenitic stainless steel electrode, such as A402 welding, repair welding of metal can be single-phase austenitic weld, but weld and base metal due to the presence of carbon diffusion between , precipitation of σ phase austenitic weld metal embrittlement, welding residual stress and during operation of the additional thermal stress and other issues, welding area increase the probability of crack defects.
c. selection of nickel-based welding consumables (ENiCrFe-3) welding ZG15Cr1Mo1V, allowing greater range of choice in the chemical composition, welding metal can get the austenite phase of Fe-based alloys and nickel-based alloys.
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