Section 9 Cladding by welding

9.1.1 Subject to special approval, the cladding of rudder stocks and pintles with a suitable corrosion-resistant alloy may be accepted as an alternative to the fitting of bronze or stainless steel liners.

9.1.2 Satisfactory service experience has been obtained from rudder stocks and pintles clad welded with austenitic stainless steels of 304L and 316L types and with Inconel 625, but the use of other types of deposited metals will be considered.

9.1.3 Automatic spiral welding, using wire or strip, is the preferred method for cladding, but consideration will be given to the use of manual or semi-automatic welding processes depositing either longitudinal or spiral runs.

9.1.4 The ends of the clad section are generally the most critical positions as they form geometrical and ‘metallurgical’ notches and can be subject to a high level of residual tensile stress. Unless suitable precautions are taken, fatigue failures at low levels of cyclic stress may occur in service at these positions. This can best be avoided by terminating the clad welding at positions well clear of zones of high stress (stress concentrations) and by tapering the weld run outs at positions well clear of the bearing surfaces.

9.1.5 Where possible, the diameter of the component should be oversize between the clad zones, so that the whole surface can be machined on completion of welding. An alternative, which is preferred for rudder stocks, is to apply the clad welding to raised diameters, the ends of which are subsequently machined, see Figure 15.9.1 Cladding of rudder stock by welding.

Figure 15.9.1 Cladding of rudder stock by welding

9.2.1 It is recommended for the majority of applications that the finished thickness of the weld cladding should be at least 5 mm and be deposited in at least two layers.

9.2.2 When austenitic stainless steels are used for weld cladding, it is recommended that the first layer should be type 309 in order to minimise dilution effects and prevent interface cracking. There is evidence that type 304L austenitic stainless steel can be prone to pitting corrosion in polluted sea-water and it is generally accepted that type 316L has better corrosion resistance in such conditions.

9.2.3 Local galvanic corrosion can occur at the ends of the clad section and where these positions are exposed to sea-water, it is essential that the junction and adjacent base material are adequately protected by a suitable means such as reinforced plastics coating.

9.3.1 Manufacturers requesting special approval for cladding are to submit full details of the proposed welding procedure which is to be forwarded to Materials Department. These should include surface preparation, welding process, type and size of welding consumables, pre-heating temperature, welding parameters, any post-weld heat treatment, final thickness of cladding plus proposals for non-destructive examination of the base material prior to welding and of the cladding in the finished machined condition. The firm should also submit details of previous experience of clad welding of the type proposed for the stated applications.

9.3.2 The facilities at the works are to be examined and the Surveyors are to be satisfied that the firm is suitably equipped and competent to undertake the required high quality of welding. It is important that high standards are maintained and that the firm continues to exert strict control and supervision of all work.

9.3.3 Procedure tests are required to demonstrate the soundness and properties of the weld cladding. Details of acceptable test samples are given in Figure 15.9.2 Weld repair test samples. The test sample should be prepared using base material complying with the requirements of Ch 5, 3 Forgings for shafting and machinery of the Rules for Naval Ships. The actual tensile strength of the material should be not less than 440 N/mm² and the carbon content should be between 0,20 per cent and 0,25 per cent.

9.3.4 Satisfactory results from these procedure tests will provide approval for the weld cladding of rudder stocks and pintles. Additional tests may be required where it is proposed to weld clad such components if the carbon content exceeds 0,25 per cent or the tensile strength is in excess of 520 N/mm².

9.3.5 Before welding, the wasted area is to be machined back to sound material and magnetic particle tested for fractures.

9.3.6 A sufficient number of weld layers should be applied to compensate for removed material, followed by one further layer.

9.3.7 Post-weld heat treatment should be applied if required, but never for stainless steel cladding on ordinary steel.

9.3.8 When components have been clad by welding, final inspection is to consist of visual examination supplemented by dye penetrant testing. This is to be carried out with the component in the finished machined condition. It is preferred that the NDE involves a sensitive fluorescent dye penetrant technique. In general, hot cracking of the deposited metal is the most likely type of defect.

Figure 15.9.2 Weld repair test samples

9.4.1 Rudder stocks and pintles weld clad by an approved manufacturer will be accepted for use in classed ships subject to the usual conditions for installation and periodical survey. It is recommended, however, particularly for larger ships, when either type 304L or 316L austenitic stainless steel has been used for cladding, that gunmetal, lignum-vitae or a synthetic bearing material be used in the bush. If stainless steel is used in the bush, it should be of a different grade and with an adequate hardness difference. The use of a ferritic/austenitic duplex structure stainless steel is recommended for the bush, but 17 per cent to 30 per cent chromium stainless steels are also suitable.