Answer MEP Question 43

Q. A single forged shaft for a small gear pump is broken in the middle. Suggest some emergency repairs to the shaft to enable run the pump. State the type of repair and procedure for carrying out the repair. What is the most viable alternative to repair if no spares are available? 

- Assuming that there is no spares available onboard and the pump has to run, the broken shaft may be repaired temporarily outboard using shipboard technics, however, the shaft has to be re ordered as an emergency spare and renewed on next possible occasion.

The possible repair technics are;

- Shrink on sleeves. 

- Orbital welding. 

- Metal spraying. 

- Arc welding process.

- Though there are many methods of repairing a broken shaft, only possible method that can be done onboard by shipboard crew and equipment are reduced to normal arc welding or MIG welding.

- Forged steel usually has a higher tensile strength but still forged steel can be welded onboard using arc, welding technics with electrodes on available the shaft can be welded using MIG welding process. 

Welding Considerations 

- Shafts commonly are made of hard steel. Welding procedures can affect repairs in many High-alloy steel is prone to cracking when Weld unless special procedures, such as preheating, are put in place. 

- The welding heat reduces the hardness and strength of the original material, but may promote the formation of a very hard layer in the he affected zone, the region bordering the weld. 

- Furthermore, the uneven heating may produce difficult-to-manage distortion so it may be preferable to introduce a mechanical joint the restores the function of the shaft, at least temporarily, until a new part is procured.

- In principle, weld repair should be attempted only on annealed material that Will be hardened again by heat treatment after welding in the us way. 

Pre-Welding preparation: 

- The key is to prepare the shaft for a welding process this involves cleaning of affected area, machining the shaft along the cracked portion.

- The broken ends shall be machined with steps or with a V groove fashion as shown in the picture below.

- The broken shaft to be held in the lathe, perfectly aligned axially Such that the total length of the shaft remains as per original size 

- Then the welding process to be started, the run has to be done on one side and then the shaft is to be rotated 180° to weld on the opposite side to avoid bending of shaft due to heat. 

- Electrode current to be set as per the size, of the electrode [3-4mm dia. current 250-300amps. Since the shaft is either of material EN57[18%Cr. 2%Ni. STAINLESS STEEL. OR EN58[ 18%Cr. 1%Ni. 3%Mo] welding electrode would be flux coated of composition C- 0.02%. Mn 0.8%. Si 0.8%. Cr 18%. Ni 11.5%. Mo 2.8% ,Frig 4-1;0%. flux shielded metal arc welding to be carried out 

Post welding: 

- Considering firstly reclamation by arc Welding; fatigue strength reductions found with welded shafts ranged from 23% to 79% compared with the un-welded state. In all cases these low strengths were a result of crack initiation at weld defects, often associated with stop/start positions. 

- Gas porosity was the most common defect, although slag inclusions, incomplete penetration, lack of fusion and heat affected zone (HAZ) cracking were also observed. 

- The arrangement of weld beads was a strong influence on fatigue strength, as a result of the greater tendency for defects to occur at, stop/start and overlap positions. Two or three-start, spiral welding was found to be preferable to either close spiral or longitudinal welding. - Preheating of the shaft tended to reduce gas porosity, to lower the susceptibility to cracking of higher strength steels, and to reduce residual stress levels.

- Post weld stress; relief by heat treatment appeared to offer only a, relatively small improvement in fatigue strength (a maximum of 25% of that of the un welded shaft, and often only 10%), whilst preheating medium carbon steel to around 300 deg C. gave a, marked reduction in residual stresses (50%,). 

Re-Dimensioning post 

- After the post Welding process is over the shaft has to be re-machined to suit the dimension of its original shape to be used in the pump back. 

- The shaft be assembled back after the dimensions are reverted. 

- The pump may be reduced with low capacity and any shock loads shall be avoided in order to avoid further failure. 

- The shaft shall be replaced upon next possible occasion.