Welding process flow and control requirements of nodular cast iron end cover castings

Since ductile iron castings are prone to crack defects during cold welding, in order to ensure that all relevant properties meet the requirements before formal welding of ductile iron castings and ensure the reliability in the process of use, the welding process summarized is certified and controlled by consulting relevant data and specifications and combining with the actual production application.

The first step is to learn to prepare welding procedure specification (WPS) according to relevant welding specifications;

Step 2: conduct internal test and formulate welding procedure qualification record (WPQR for short);

The third step is to submit WPS and WPQR if necessary for qualification certification and confirmation by a third party;

Step 4: when welding again, qualified welding personnel must carry out welding, and third-party qualification certification can be carried out;

Step 5: complete internal operable welding operation instructions according to relevant specifications, WPS and WPQR;

Step 6: regularly review the welding process if necessary to ensure that the welding process meets the process requirements.

For the welding repair and repair of nodular cast iron castings of the end cover, after the WPS is prepared according to the above steps, the relevant test design is carried out by consulting relevant standards and data, and clear welding process and qualification requirements are formulated from the aspects of electrode selection, groove selection, welding current and interlayer temperature control, welding sequence and number of layers, detection methods before and after welding, post welding heat treatment, etc. The requirements for control points of relevant process parameters in the welding repair process of nodular cast iron are as follows:

(1) Welding material selection

Enife CL cast iron electrode is used for the welding of nodular cast iron end cover, and the amount of electrode w (Ni) is 45% ~ 60%. Compared with pure Ni electrode, the weld with ferronickel electrode has better strength and ductility, and its mechanical properties can meet the basic performance requirements of nodular cast iron (as shown in Table 2). Therefore, it is more suitable for welding nodular cast iron. Before formal welding, the welding rod shall be baked in a drying oven with a temperature of 150 ℃ for at least 1 h. when in use, it can be taken out of the drying oven and stored in an insulating cylinder with a temperature of 40 ~ 60 ℃ for standby.

(2) Welding groove selection

When repairing the nodular iron end cover casting, according to the thickness of the defective part of the nodular iron end cover casting and the welding material, the V-shaped or U-shaped groove is usually used, and the groove size is designed to be 30 ° ~ 60 °. As the defects of nodular cast iron castings generally do not penetrate the whole wall thickness, it is recommended to adopt U-groove (as shown in Figure 1) during welding repair. The groove design shall be able to meet that the welding material can contact the bottom of the groove and reduce the width of the welding area, so as to ensure good fusion and shrinkage stress release. The groove design shall adopt upward taper to avoid too steep welding start and weld bead.

(3) Welding process requirements

For ductile iron end cover castings, the welding current is generally controlled at 95 ~ 150 a during welding repair, and the intermediate temperature during welding shall be controlled at ≤ 65 ℃. For each layer of weld, the sharp hammer hammering method shall be used to moderately repeat hammering on the weld until it is full of pitting, so as to remove the possible welding residual stress and avoid post welding stress. At the same time, in order to avoid the potential crack risk caused by welding stress, the ductile iron end cover blank castings can be heated slowly (the heating rate is less than 65 ℃ / h) to 550 ~ 600 ℃. After holding for 4 hours, the cooling rate below 55 ℃ / h can be reduced to below 200 ℃ for air cooling for post welding heat treatment. For the ductile iron end cover castings after finishing, due to the special requirements of the processing surface, Stress relief aging cannot be carried out. Therefore, after welding, the welding defect parts shall be covered with thermal insulation materials to cool the weld area slowly to reduce stress.

(4) Determination of welding sequence

When one or more welds are repaired, several welds must overlap each other without interruption. The last weld shall be welded on the previous weld instead of the base metal. During welding, the welding sequence of the weld shall comply with the welding sequence shown in Figure 2.

(5) Operation requirements for welding repair process

Before formal welding repair, the tools to be used during welding repair shall be prepared. Since the defects of nodular cast iron will not penetrate the whole wall thickness during defect welding repair, the defects of nodular cast iron need to be removed by rotating file until the metal surface is exposed. There is no oil, grease, dirt, rust or other foreign impurities through visual inspection. If any problems occur, they must be removed by using corresponding solvent or heating evaporation method, Until the visual inspection is qualified. If the heating evaporation method is adopted, the oil can volatilize at a temperature above 398 ℃. In order to ensure that the defects are removed completely, the magnetic particle flaw detection method shall be used to inspect the defects before welding. After the inspection, the dry powder shall be blown away by high-pressure air. Before welding, penetrant testing shall be avoided to avoid potential welding pores and other defects caused by incomplete removal of penetrant. In the welding process, adjust the current to the specified range, confirm the welding sequence and weld layer by layer. Swing welding shall be prohibited in the welding process, and the final weld meat height shall be 2 ~ 3 mm higher than the base metal to ensure the size and performance requirements of nodular cast iron castings.

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