A. Hard Surfacing

1. Hard surfacing is the application of a durable surface layer to a base metal to impart properties like resistance to impact wear and erosion or pitting and corrosion or any combina­tion of these factors. Hard surfacing can be applied by arc welding.

2. Hard facing materials for wear resistance tend to suit specific types of wear like abrasive or sliding wear or build desired dimensions.

3. Electrodes used for such applications are called hardfacing electrodes, covered by AWS A 5.13—1970 used as surface filler metal for gas and TIG welding, and coated electrodes for arc welding.

Type Hardness range BHN A 250—280 (Hard) B 350 — 380 } (Harder) C 280 — 320 J (Harder) D 600-625 (Hardest)

4. The hardfacing electrodes are designated on the basis of hardness of weld deposit e. g.,

Applications

R Moderate hardness: used in T gears/machine parts.

J Brake shoes, cams, rollers,

T large wheels.

R Metal cutting/forming tools, punches,

T dies, crushers, hammers crane wheels.

The above electrodes A, B, C and D give martensitic deposit and impart hardness in as - weld condition at normal cooling rates in air.

5. To obtain desired results for a specific application it is necessary to understand the effect of base metal dilution and cooling rate on the hardfacing deposit. Base metals having high carbon and hardenable elements like Cr and Mo are likely to develop underbead cracks, due to hydrogen from the rc. Low hydrogen, hardfacing electrodes are to be used in such cases.

6. Hardfacing deposits respond to mechanical and thermal treatments. The operation introduces distortion which can be countered by proper fixturing, bead sequencing and pre­heating the base metal.

7. Hardfacing materials may be classified as follows.

(a) Alloy steels (Cr, Ni, W and Mn) : Austenitic or martensitic are available in the form of electrodes. Martensitic deposits may be heat treated to get desired properties.

(b) Complex alloys (stellite) are used as cast rods or flux coated electrodes. Mainly used in wear resistance applications.

(c) Tungsten carbide (one of the hardest materials) used for cutting tools.

8. Semi-austenitic alloys provide balanced composition of good wear and impact re­sistance and is most widely used of all hardfacing materials. These are iron based alloys con­taining upto 20% alloying elements C = 0.1-0.2% and Cr = 5-12%). The deposit, if cools slowly gets time for austenite to transform to martensite and is less ductile, if cools fast by using short beads, gives soft and tough austenite.

9. Austenitic Mn-steels are used to built-up worn Mn-steel parts. They are used where resistance to severe impact and abrasion are required.

10. Austenitic stainless steel deposits provide resistance to corrosion and chipping from repeated impact forces. Protect turbine blades from corrosion and cavitation erosion. Also used as buffer layer for other hardfacing materials to avoid brittle bond.

11. Tungsten carbide deposits are suitable for cutting tools, tools for earth and rock cutting, chromium carbides used for hard surfacing when corrosion resistance is also required.

12. Hardfacing processes and applications. (Slow cooling rates prevent underbead cracking).

Processess Applications Precautions if any

1. Oxy-acetylene Hardfacing, Cracking is minimised by flame pre-heat­

ing used for small delicate parts requiring thin layers.

2. Manual Metal Arc Common for repair hard facing. Gives deep penetration

deposits.

3. TIG 4. MIG 5. SAW

Requires little pre-heating, used for high alloy steels, Cr and stainless steels, Ni-base alloys, Copper and Co-base alloys. Aust-Mn. steels.

Often used for cladding and build-up. Not very common for hardfacing. Specially suited for aluminium bronze overlays.

Good wear resistance with single layer. DCRP low depo­sition rate and thin beads. DCSP gives high deposition rate and thick deposits.

3. The major problem in hardfacing is the peeling-off of the deposited layer, particularly when the base metal contains less than 0.15 per cent carbon. Preheating the base metal and slow cooling will reduce peeling tendency and underbead cracking. Spalling can be avoided by :

(a) cleaning base metal surface (b) preheating base plate and slow cooling (c) depositing thin layers and peening each layer to relieve stresses.

B. Cladding

1. Cladding, is similar to hardfacing, but is normally a corrosion resistant overlay. In high pressure applications such as nuclear reactor vessels, cladding provides a combination of

mechanical properties and corrosion resistance. Cladding of low alloy steels with austenitic stainless steels is quite common in nuclear reactor vessels.

2. Cladding Processes and applications

Cladding Processes Applications

1. SAW Most of cladding is carried out. Alloy addition is through

flux, high deposition rate ; Slow welding decreases dilu­tion (1.2-5 mm/s)

2. Plasma Cladding Well controlled heat input, independently controlled

deposit thickness and penetration, high weld purity, clads difficult to weld metals where SAW Fluxes developed, and increased productivity.

Surfaces which are deposited by cladding technique include:

1. Austenitic stainless steels

2. Inconel

3. Nickel and cupro-nickel

1. SAW 2. Plasma cladding Fig. 7.5 Gas metal plasma hot wire process 3. Cladding integrity While cladding with austenitic steel on reactor vessels to protect the underlying steels from corrosive environments, ensure that the deposit microstructure contains austenite plus only 3-10% ferrite to avoid solidification cracking. Dilution of deposit may take place when using SAW. SMAW electrode E 309 (23 Cr-12 Ni) to avoid dilution. Cracking in cladding may expose base metal to corrosive environment. Sometimes the cracks may penetrate the base metal. Causes of cladding degradation are : — microstructural/phase changes, sensitization, embrittlement, sigma phase formation,

loss of corrosion resistance.

- low cycle fatigue cracking due to thermal loading.

- carburization and subsequent sensitization.

- loss of adherence (fusion).

- hydrogen embrittlement of weld overlay during shut down and restart.

- stress corrosion cracking due to chlorides and polythionic acids, principally during nuclear vessel shut down periods.

Sigma phase formation can be minimised by keeping the ferrite content of the cladded stainless steel in the range of 3-10 percent. Ferrite phase serves to nucleate sigma phase during post weld heat treatment which increases chances of steel to hydrogen embrittlement.

Embrittlement of austenitic stainless steel cladding material during post welding heat treament is due to both the sigma phase formation and carbide precipitation and is minimised by using low carbon material and by keeping ferrite content at the lower end of the safe ferrite content range.