Welding of Creep-resistant Steels

Welding of chromium-molybdenum-vanadium creep-resistant steels requires skill, practise, not to mention knowledge. This article contains some practical pieces of information about work needs to be done before welding, during the process and afterwards.

This kind of steel can be weld using all kinds of arc welding methods, SMAW or GTAW, but it can't be welded with oxygen-acetylene gas.

Before welding

Succesful welding of creep-resistant steels requires considerable preparation. The electrodes, the working area and the piece itself must be prepared.

Electrodes

Electrodes must be dried up in a furnace directly before the process. It is recommended to:

• hold rutile electrodes for two hours at 120-150°C.
• hold low-hydrogen electrodes for two hours at 120-150°C.
   

Do not use expired electrodes.

Working area

Welding should be performed in steady air, no wind or raindrops are acceptable. Do not weld when it's colder than 10°C. Use heaters, covers and asbestos sheets to ensure proper working conditions in the area.

Preheating

The steel piece must be preheated before cutting, bevelling and welding. Incorrect preheating or no preheating at all may result in various adverse effects, including severe internal stress, hardening of welding area and cracking.

The preheating temperature for chromium-molybdenum-vanadium steels usually ranges from 250 to 300°C. The recommended heating rate is 30-50°C per hour. What area must be preheated? There are two options:

• to preheat the whole piece (usually in a furnace)
• to preheat the welding zone (usually by means of heating elements or heating covers)
   

To choose between those options take into account nature and extend of the welding, the thickness and dimensions of the piece, the length and shape of the weld, the transport capacity and similar factors. Take sufficient time heating up, because the piece must be heated through the entire profile. If You choose to heat only the welding zone, remember that it must be at least 50mm wide from each edge.

In case of some steels both temperature and heating details may vary, so always read the datasheet provided by the manufacturer.

Edges

As we've already mentioned, the piece must be preheated to be suitable for cutting or bevelling. After thermal cutting grind 3mm deep. Cut surfaces must be closely analysed and any cracks or defects are unacceptable.

Welding

Let's discuss the welding process itself.

Single-run welds are to be avoided with this type of steel. If the piece is thinner than 6mm, weld two times from two sides. Welding multiple times is important because the result of adding a new weld is that the bottom one is toughened. Ones the welding is started it must be carried out without breaks until the process is complete.

Tacking

Tack carefully, as the positional welds (or tacks) will be a part of a future weld. Remove defective tacks.

Root pass and welds

Begin every layer with beads on the sides, then with filling beads - this rule must be carefully observed. For side runs use coated electrodes or electrode wires 2.5mm thick, and for filling runs - 3.25mm ones. The welds should not be very thick, because no toughening of underneath welds would be achieved.

The electric arc should be ignited on the side, which requires the correct connection of the piece. Ignition in a wrong place may result in almost instant hardening of the surface of the piece, which leads to cracks. Keep the electrode perpendicularly toward the surface. The tungsten electrode must not touch the wire or the molten pool at any moment.

Weld slowly to allow the gas to escape from the pool. Reduce the speed even further at the beginning and at the end of each run. It is also recommended to re-run both the beginning and the end of each bead. Be especially careful during the root pass.

Do not begin nor end a run directly on top of the beginning or end of another bead or welding tacks.

After welding

After removing the clamps grind and sand the surface of the weld.

Let the piece cool down. Cover the piece with asbestos covers to slow the cooling process. Now heat treatment is necessary to obtain good mechanical properties for the weld and to dissipate internal stresses.

Source: Ryszard Pasierb, "Spawanie żarowytrzymałych stali chromowo-molibdenowo-wanadowych", ISBN 83-204-0381-2