Sulfur impurities in the weld metal and phosphorus on the performance of welded joints have a greater impact, and thus in some important engineering structures you want to be limited [1,2]. The weld metal in manual arc welding of sulfur, phosphorus, mainly from the skin of steel and Welding Electrode Supplier rod drugs, drugs in the welding process, electrode melting occurs when the skin will affect the weld metal metallurgical reactions of sulfur and phosphorus content. To this end, study the alkalinity of the skin electrodes drug desulfurization, dephosphorization behavior, with a view to the skin electrodes drug formulations designed to provide information.

Test material and methods

1.1 Test Materials
Selection of welding core of H08A φ4 mm steel core, and its chemical composition (mass fraction,%) were: C 0.07, Si 0.017, Mn 0.44, S 0.018, P 0.01. Use of alkaline hydrogen from low alkalinity slag pressed into different welding electrode to communicate test electrode diameter of 6.4 mm.

1.2 Test Method
AC arc welding by hand, welding speed of 170 mm / min or so, the Mig Welding Electrode current of 175 ~ 185 A, arc voltage of 21 ~ 25 V. Deposited metal and slag sulfur, phosphorus analysis using conventional methods. Skin electrodes drug alkalinity (alkalinity are involved in this article refers to the alkalinity of the skin electrode drugs) by the International Institute of Welding (IIW) formula recommended B1 alkalinity.

2 results

Electrode in order to obtain basicity of the sulfur deposited metal and phosphorus content of the laws affecting the use basicity of different electrode welding tests carried out, analysis of the deposited metal in the slag and sulfur, phosphorus, and Accordingly derived sulfur and phosphorus in the slag and the deposited metal distribution coefficients in Table 1. Table Ls, Lp [Ls = w2 (S) / w1 (S), Lp = w2 (P) / w21 (P)] are the sulfur and phosphorus in the slag - molten iron in the distribution coefficient, w1, w2, respectively, on behalf of a elements in the deposited metal and slag mass fraction.

3.1 basicity electrode on the desulfurization behavior
The data in Table 1 can be made basicity electrode B1 and Ls and w1 (S) value of the relationship between the curves. , Ls with the basicity of the electrode increases, but when alkalinity reaches a certain value, Ls worse. Sulfur content of deposited metal variation just the opposite, and can be seen in the experimental conditions, by changing the electrode method of basicity to desulfurization is limited.

In the Subway Construction, the tool wear serious, serious impact on the construction schedule and cost. By selecting the appropriate surfacing materials, improved blade surface properties, effectively extending its life.

Line 5 for the first time shield construction method is applied to Beijing subway tunnel construction, subway construction of key equipment shield is the shield tunneling machine. Beijing Subway Line 5 construction using earth pressure balance shield machine is costing 48 million yuan or so from Germany to introduce the latest large-scale machinery. The test section of the underground tunnel shield construction, accumulated a wealth of experience and experimental data, also showed some problems.

Shield machine parts also need to import, but as a shield of consumables is a tool to affect the construction schedule and cost is an important issue. Focusing on improving the life of the shield machine tool research, so as to realize the localization of the shield machine tool has become one of the issues need to be solved. Beijing University of the relevant group on the shield machine tool repair and localization were studied. Tool edge method of choice for repair welding; material selection, combined with the group's research experience and shield machine tool performance requirements identified as flux-cored Welding Supplies.

Beijing Subway Line 5 shield test section is sand and gravel formations, large gravel content. Formation of up to 45% quartz content, the tool wear faster. TBM cutter diameter 6.2m, outermost cutter cutting tool maximum speed of up to 1m / s, a great impact around the knife edge. Through the above analysis, the construction sand and gravel stratum, the shield machine tool main failure in the form of impact damage and abrasive wear.

Head shield not only in the construction process gravel suffer wear and tear, also suffered a strong impact. This requires the surfacing alloy not only has high hardness, but also has good toughness, a strong impact in the case of repeated less prone to fall off.

Surfacing material selection

Iron-based hardfacing alloys vary widely, with good toughness and wear resistance, can meet different needs, and low prices, this study identified the use of iron-based alloy. For iron-based alloy is, C is the most important element, C content within a certain range, with the surfacing layer hardness and wear resistance increases with C content, impact resistance weld overlay also will be improved, but C content is too high, when the impact toughness of weld overlay by the poor performance is particularly strong impact there will be more cracking and spalling. Cladding layer in order to prevent cracking and spalling of the impact of this study to restrict the cladding layer in the carbon content, and so has the use of Mo grain refinement, combined with the ability of carbon alloying elements to improve overall performance cladding layer, while adding Ni to improve the toughness of weld overlay. To ensure the surfacing layer of abrasive wear resistance and impact resistance, to determine the Cr-Ni-Si-Mn-Mo alloys to strengthen multi-core drug formulations as the main component.

To ensure the mechanical properties of welding wire, welding flux cored wire core composed mostly of traditional Chinese medicine powder alloy (mass fraction 75% to 80%). Contains a small amount of flux-cored cored arc stability and slag forming agent agent (mass fraction 20% to 25%), mainly used to ensure performance of flux-cored welding processes, including welding arc stability, weld, off slag, spatter, welding fumes and so on.

In general, the mechanical properties of weld overlay to meet the requirements under the premise of nitrogen, hydrogen, oxygen, sulfur and phosphorus content of no special requirements, but out of condition and welding requirements, requirements for weld strict control of the crack, that crack-free weld surface after welding. Department of alkaline slag splashing, slag removal system difficult to control than the acidic residue and acidic than alkaline slag arc stability, weld appearance. In addition, the amount of dust on the welding, the slag alkaline dust and hair were higher than the rate of dust acidic slag. Therefore, determination of selected acidic slag slag. In the flux-cored cored formula design, add mineral powder are rutile, quartz, fluorite, iron oxide, zircon, wollastonite.

Conditions by the shield shows the tool, the tool is necessary to have a high hardness to withstand the high stress abrasive wear, but also have sufficient resilience to absorb the impact of sand and gravel. So select the Cr-Ni-Si-Mn-Mo as the main component of Mig Welding Electrode, the specific composition in Table 1. No warm-up before welding wire after welding without heat treatment, welding crack-free surface layer.

Cladding layer organization primarily for low-carbon lath martensite, the martensite from austenite at high temperatures in the cooling process of change from the original austenite grain boundaries have not changed completely austenitic body residues.

Cladding layer performance

Samples with the developed flux-cored wire for welding, to remove the base material of the cladding layer composition, the impact on the performance of the test when three or more welding. Rockwell hardness of cladding layer as shown in Table 2.

The impact of the shelves were used in the homemade quality of 3.5kg and 7.5kg of shot 1m height on the specimen by the impact, the impact of the number were 10, 20 and 30. The impact of changes in hardness as shown.

Cladding layer by the Rockwell hardness changes visible, with the impact, the number of weld hardness increasing trend, but the upper limit of hardness. And shot more than 10 times the impact of cladding layer hardness can be close to the upper limit (HRC58.5 so), indicating the number of welded layer with less impact on the performance of a significantly strengthen the impact of the phenomenon, which in the shield machine tool service during the great significance: the impact of a few times a pebble can make the shield machine tool show higher hardness, thereby rapidly increasing the wear resistance of cutting tools.

Through a series of pilot study shows that the welding flux-cored welding process well-welded hardness up to HRC55, no warm-up before welding, after welding without heat treatment, welding layer without cracks.