ln the process of ship building, the babbitt metal is widely used in all kinds of pillows on ships. In ship repair, in order to improve the repeated utilization rate of babbitt metal pillow and reduce the capital and time of outsourcing recasting, according to the practical experience of repairing babbitt metal pillow in the past more than 30 years, a set of welding repair technology of babbitt metal with high qualified rate is summarized.

1 Introduction

Many rotating equipment in ships rely on the support of various bearings and lubrication of bearings by lubricating oil to work. The intermediate bearing bushing of the ship’s tail shaft, the connecting rod bushing of the main engine, the bushing of the generator, etc., are all made of Babbitt alloy. Due to vibration or failure of the oil supply system during long-term operation, the Babbitt alloy on the bushing is worn, and even causes the Babbitt alloy to fall off and burn. Therefore, casting and repair welding are often used in repair. This article will introduce the successful practice of TIG welding repair technology for damaged and damaged bushings.

2 Introduction to Babbitt Alloy

2.1 Characteristics of Babbitt Alloy

Babbitt alloy has high wear reduction performance, good embedding, friction compliance and shaft resistance. Hard phase particles are evenly distributed in the soft phase matrix. The soft phase matrix gives the alloy good embedding, compliance and anti-bite properties. After running-in, the soft matrix is ​​concave and the hard points are convex, so that a small gap is formed between the sliding surfaces to become an oil storage space and a lubricating oil channel, which is conducive to reducing wear; and the convex hard particles play a supporting role, which is conducive to bearing.

2.2 Commonly used babbitt alloy models

Most of the ship’s tail shaft intermediate bearing bushings, main engine connecting rod bushings and generator bushings use two types of babbitt alloys, namely ZSnSb11Cu6 and ZSnSb8Cu4, as shown in Table 1.

2.3 Defects and damage forms of babbitt alloys

The main damage forms of the ship’s tail shaft intermediate bearing bushing (babbitt alloy) are as follows:

(1) Local defect or wear

Due to the long-term operation of the bushing, the babbitt alloy layer on the bushing is worn and detached due to vibration, as shown in Figure 1.

(2) Completely broken or delaminated

If the oil supply system fails, burning will occur, and both the upper and lower bearings will be burned and broken, especially the lower bearing, where the Babbitt alloy layer will even be delaminated. This kind of serious damage cannot be repaired by welding, and needs to be repaired by recasting.

3 Materials and welding characteristics of Babbitt alloy

Babbitt alloy is a soft metal material, which is usually repaired by recasting and welding. Since Babbitt alloy has a low melting point (240°C) and strong fluidity, the tin liquid in the molten pool is easy to lose, so it is difficult to cast or weld. Through continuous practice, new repair methods and processes that are simpler than traditional ones have been explored. The following introduces the repair method of TIG welding when the damage is serious.

3.1 Material characteristics of Babbitt alloy

Tin-based solder is a soft solder with a low melting point. It can be melted at a relatively low temperature by brazing, and the nodes to be welded can be connected. It is a method of providing continuous thermal and electrical conductivity, or used for sealing liquid and gas containers, and the solder joints are not subjected to large stress.

Soft solder should meet the following requirements:

(1) Have a certain thermal and electrical conductivity;

(2) Maintain the required strength between the connecting parts below 200 ℃;

(3) Have a dense structure and good sealing;

(4) Have good wettability between the soft solder and the soldered parts and basic materials.

The thermal and electrical conductivity of soft solder is poor, only 8%~15% of copper. However, there is no obvious resistance (such as resistance) in the road (such as the circuit), because the conduction path is short and the contact area at the solder joint is large.

The quality of the solder joint depends on the nature of the surface to be soldered, the properties of the soft solder and the choice of flux. In fact, it depends on the wetting process of the molten soft solder on the solid metal surface to be soldered. Tin is an active element in many soft solder components. It can wet and fuse with the base metal to be soldered, such as Cu, Fe, Ni, etc., to form a very thin layer of metal compounds.

The use of flux is to clean the metal surface to be soldered to avoid affecting the wettability. The main component of the flux is ZnCl2, which produces free hydrochloric acid in the presence of water. When soldering copper, the oxide layer dissolves into chloride and leaves the base copper, and the molten solder gradually spreads on the copper.

3.2 Soft solder composition and properties

Soft solder is generally a Sn-Pb alloy with a eutectic composition of 26.1%Pb and a eutectic temperature of 183 ℃, which can ensure a low soldering temperature and avoid damage to temperature-sensitive components.

When soldering by hand, choose Sn-50%Pbd alloy. As the temperature decreases, the solubility of Sn in Pb decreases, Sn precipitates, and the solder softens; in Sn-Pb-Sb alloy solder, the precipitation of SnSb intermetallic compounds is particularly obvious; Sn-5%Ag and Sn-5%Sb alloys can not only maintain the strength of the solder to 200 ℃, but also have similar wettability to eutectic alloys.

For solder used at low temperatures, high Pb alloys should be selected, such as Pb-10%Sn or Pb-5% Sn-1.5%Ag alloys. The wettability and strength of this alloy will be affected, but Sn will not undergo phase changes at low temperatures (such as 173K), resulting in serious loss of solder plasticity and impact strength.

In these solders, 0.001% Al will cause oxidation, and the aluminum oxide film will affect the wettability at the interface between the liquid solder and the flux; the solder generally contains 0.1%~0.5% Sb, and the creep-resistant solder can reach 5% Sb. A small amount of antimony (0.1%~0.5%) can improve the wettability of Pb-Sn solder to brass. Adding 0.1%~0.25% Bi can increase the spreading speed of eutectic Sn-Pb solder. When the Bi exceeds 0.5%, the solder surface will change color.

Cadmium will reduce the wetting speed, and its oxide film will darken the solder surface and cause brazing defects; copper has little effect on the wettability of the solder, but when it exceeds 0.25% Cu, it will affect the appearance of the brazing surface due to the formation of Cu-Sn compounds; phosphorus exceeding 0.01% P will affect the wettability of the solder on copper and low-carbon steel; sulfur (S) affects the appearance of the brazing surface, and the S content in the solder is limited to within 0.001 5%; Zn is easily oxidized to produce oxides, and the solder surface quality deteriorates when it exceeds 0.003% Zn. Therefore, the combined effect of various impurities cannot be underestimated and should be strictly limited.

3.3 Difficulties in the repair process of babbitt alloy

Previously, welding repairs were mainly repaired by traditional wind brazing or high-power electric chromium iron. These repair methods have the following defects:

(1) Production of welding wire

It is necessary to make a homemade welding rod and use an oxygen-acetylene flame to directly heat the block of babbitt alloy. Its defects are: on the one hand, when heated and melted, the welding wire liquid that flows out will solidify immediately, becoming welding wires of different sizes, with thick and uneven diameters; on the other hand, because the babbitt alloy is directly heated by an oxygen-acetylene flame, the impurities contained in it cannot be removed and will also solidify into the welding wire, making the resulting welding wire very rough. It is difficult to melt the filler material in traditional wind brazing or high-power electric chromium iron repair;

(2) Repair effect

The traditional gas welding method for welding and repairing bearings cannot meet the requirements of repair welding: ① Use a wind lamp to directly aim at the bearing. Although the melting power meets the requirements of repair welding, it will damage the intact part adjacent to the parent body or the repair part, and the welded part and the intact part cannot be melted together; ② Use a wind lamp to heat a hammer made of pure copper without heating it, and use the hammer to conduct heat for welding. This will dissipate heat quickly, resulting in cooling and failure to melt to achieve welding. It is also difficult to melt the welded part and the intact part, and there is often undercut at the joint; ③ Use a high-power electric chromium iron for welding, with a temperature of 500 A. Taking the electrochromium iron as an example, the welding of pores and small-area bearings with thin walls is acceptable, but for thick-walled bearings, the temperature is not enough, the melting power cannot meet the repair welding requirements, and the joints often have undercuts.

4 Repair method using TIG

For small-area damage and defects of Babbitt alloy bearings, conventional welding repair methods include oxyacetylene brazing and soldering iron welding. Oxyacetylene brazing and soldering iron welding are prone to undercuts, incomplete penetration and pores. In particular, the oxyacetylene brazing operation process is complicated and easy to damage the matrix.

The following introduces a completely different welding repair method for Babbitt alloy bearings. It is not only simple to operate, but also does not require flux, simplifies the repair process and has high welding quality. The qualified rate after repair can reach 100%, overcoming the defects of undercuts, incomplete penetration and pores that are easy to produce by oxyacetylene brazing and soldering iron welding, and the life of the bearing after repair is extended; it can be applied to thicker damage on Babbitt alloy bearings, saving costs and improving production efficiency.

Based on the experience of repairing babbitt alloy bearings over the years, the TIG welding repair method stands out among many methods. The specific process steps of TIG welding babbitt alloy are introduced as follows.

4.1 Preparation before welding

(1) Preparation of welding wire

The material of the bearing is babbitt alloy, model ZSnSb11Cu6 and ZSnSb8Cu4, which is a soft metal with a low melting point.

Select matching babbitt alloy materials for melting (small crucible) to make homemade welding wire. The welding wire melted in the small crucible is relatively pure, which can remove the impurities inside and remove the floating objects suspended on the surface; tilt the ∠ 30×30×2 stainless steel angle steel so that the angle between the stainless steel angle steel groove and the horizontal plane is 20°~40°, then use a small iron spoon to pour the molten babbitt alloy liquid into the stainless steel angle steel groove, rotate the stainless steel angle steel, and collect the welding wire that falls off the stainless steel angle steel.

(2) Treatment of the bearing surface

The bearings that have been in lubricating oil for a long time have oil molecules that have penetrated into the body. During welding repair, these leaking oils will hinder the fusion of metals, so they should be carefully cleaned.

First, determine the location of the welding repair and clean the bearings ultrasonically. If conditions are not met, use metal cleaning agents to clean the oxide film and oil stains on the surface. Then keep the bearings clean and immediately perform welding repairs.

4.2 Welding repair process

(1) Use TIG DC welding: use argon protection, the argon flow rate is 8 to 10 L/min, the electrode diameter is 3.2 mm; a small ceramic protection nozzle; use a headband photochromic mask, and be gentle when holding the welding wire;

(2) Use flat welding and left-hand welding method: do not rush to fill the bottom layer of the weld, first start the arc in the welding area, because the old bearings have infiltrated a lot of lubricating oil during use, and it cannot be completely removed after cleaning. When welding, repeatedly start the arc back and forth in the welding area, using TIG Use arc light to force out the oil molecules inside; then use a clean rag dipped in a little acetone to wipe off the oil molecules floating on the surface; finally use a wire brush to brush off the oxides floating on the surface, and then perform wire filling repair welding;

(3) The melting point of Babbitt alloy is relatively low. When starting the arc, the electrode should be correctly aligned with the welding area, and the arc pressing method should be used to prevent the Babbitt alloy in the non-welding area from melting; the welding wire should be made as thin as possible to facilitate the arc pressing operation during welding;

(4) When welding, use a photosensitive color-changing mask to accurately feed the wire and adjust the welding machine to delay gas shutoff; when each welding arc is closed, do not immediately remove the nozzle from the welding area so that the delayed gas can effectively protect the area to avoid causing pores; pay special attention to the fact that there should be no wind during welding, and take wind-blocking measures if necessary;

(5) The surface of the last layer of weld should be slightly higher than the original surface of the bearing, and pay attention to not producing undercuts and unfused defects at the junction with the original surface, and finally obtain a smooth bearing through machining. Figure 2 shows the bearing surface after TIG welding repair.

5 Repair effect

In order to verify the repair effect of the bearing in this paper, the author selected the same bearing, and artificially damaged it with a scratch area of ​​3 c㎡ and a depth of 2 mm, a damage of 5 mm, a defect of 12 mm, a loss of 30 mm, and a loss of 35 mm, and then repaired it. The test results are listed in Table2.

It can be seen from Table 2 that the traditional bearing repair method is limited to minor repairs; while the bearing repair method in this paper can be applied to the repair of thicker damaged babbitt alloys, and the repair thickness can reach 35 mm, and the repair effect is best for bearing damage not exceeding 30 mm in thickness.

Babbitt alloy is widely used in various types of bearings on ships, and its quality is related to the normal operation of the ship’s main engine, generator and tail shaft. When repairing ships, casting and TIG welding of babbitt alloys will produce high-quality products. In the comparison of various welding methods for repairing babbitt alloy, TIG welding is currently the simplest and most ideal welding method.