Alloy Steel Welding

7/29/2019 Alloy Steel Welding

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Welding of alloy steelWelding of alloy steel

Introduction to Alloy Steel

- Alloy steel contains elements such as chromium, nickel,vanadium, molybdenum, tungsten, cobalt, boron and copper;and manganese, silicon, phosphorous and sulphur inamounts greater than normally are present.

- The purpose of adding alloying elements in to steel is toachieve

o Strengthening of the ferriteo Corrosion resistanceo Better hardenabilityo Grain size controlo Improved machinabilityo Improved high or low temperature stabilityo Improved ductility, etc.

- Given below is the composition of typical alloy steel.

C 0.2 0.4 % Mn 0. 1.0 %Si 0.3 0.6 % Ni 0.4 0.7 %Cr 0.4 0.6 % Mo 0.15 0.3 %Fe Balance

1. Carbon in steel affects hardness, tensile strength, machinabilityand melting point.

2. Manganese contributes markedly to strength and hardness. Itlowers both ductility and weldability, if present in high percentagewith high carbon content in steel.


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3. Silicon improves oxidation resistance and strengthens low alloysteels.

4. Nickel increases toughness and resistance to impact. It rendershigh chromium iron alloy austenitic. It strengthens steels andlessens distortion in quenching.

5. Chromium adds to depth hardenability with improved resistanceto abrasion. It helps preventing corrosion and oxidation.

6. Molybdenum promotes hardenability of steel, makes it finegrained, counteracts tendency towards temper brittleness, raisestensile and creep strength at high temperature, etc.

7. Vanadium promotes fine grains in steel, increases strength whileretaining ductility, etc.

8. Tungsten Improves hardness and strength at high temperatures,resists heat and promotes fine grain.

9. Cobalt contributes to red-hardness by hardenings ferrite.

10. Copper (0.2 0.5 %) when added to steel increases resistance toatmospheric corrosion and acts as a strengthening agent.

11. Aluminium produces fine austenitic grain size and acts as a de-oxidizer.

12. Sulphur imparts free machining properties.

13. Boron (0.001 0.003 %) is a powerful hardnebility agent.

14. Titanium reduces martensitic hardness in chromium steels.

- Alloy steels can be classified as

a. Low alloy steels (total alloy content up to 5 %) b. Medium alloy steels (total alloy content up to 5 to 8 %)c. High alloy steels (total alloy content up to 8 %)


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- Some of the popular alloy steels are

i. Low alloy, high strength steels (a typical composition isC 0.12%, Mn 0.60%, Si 0.30%, Ni 0.55%, Fe balance).

ii. Chromium steels

iii. Nickel steels

iv. High nickel-chromium steels

v. Low carbon molybdenum steels

vi. Tool steels

vii. Stainless steel.

Welding of Low Alloy High Strength Steels

Introduction

- Low alloy steels (under 0.3% carbon) are 10 to 30 %

stronger than the straight carbon steels and are used whereresistance to corrosion and heat is desired.

Low alloy steels are of course slightly more expensive thanstraight carbon steels.

- Good resistance to atmospheric and other mildly corrosiveenvironments characterizes low alloy high strength steels.

- Such steels have yield strength values between 50,000 and80,000 psi (3500 and 5600 kg/cm ) and tensile strengthvalues between 70,000 and 110,000 psi (4900 and 7700kg/cm ).

- Such steels may find applications as sheets and thin plates intrucks, railroad cars, road building equipment, etc.


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Weldability

- Weldability of low alloy steels is dependent upon thecomposition and the hardenability; those exhibiting low

hardenability being welded with relative ease, while those of high hardenability require preheating and post heating.

- Welding of such steels is carried out on mush the same linesas that of carbon steels of equivalent carbon contents.

- Sections of 6 mm or less may be welded with mild steelfiller metal and may secure joint strength approximating

base metal and weld reinforcement.

Alloys of higher strength require metals of mechanical properties matching the base metal.

Special alloys with creep resistant or corrosion-resistant properties must be welded with filler metals of the same chemicalanalysis.

- An important consideration in welding many high strengthlow alloy steels is the prevention of under bead or cold

cracking; which can be minimized by using low hydrogentype electrodes (either mild steel or alloy-steel analysis) anda slower rate of cooling.

Welding of low alloy high strength steels which high-hydrogen type of covered mild steel electrodes, however, usuallyrequires that the assembly be preheated.

Welding with low hydrogen electrodes generally does notrequire preheating except for highly restrained sections.

Welding Process

- All the common welding process can weld the low alloyhigh strength readily.

- However, low alloy high strength (hot-rolled) steels have thefollowing chemistry limitations:


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a. For resistance welding, 0.12% C max and 1% Mn max.

b. For other welding process, 0.2% C max, 1.25% Mn max,0.05% S max, 0.15% P max and 0.90% Si max.

1.Oxy-acetylene Welding

- The type of filler rod employed depends upon themechanical properties required. A high tensile steel rod will

prove effective. For corrosion resistance, etc. the weld metalmust match with the parent metal.

- A flux is used to counteract the oxidation of alloyingelements.

- After welding, a post heat-treatment is necessary for the heattreatable low-alloy steels to refine the grain structure.

2. Flux-shielded Metal Arc Welding

- Mild steel electrodes will work very well with steels havinga carbon content under 0.14%. Weld develops tensilestrength as high as 80,000 psi (5600kg/cm ) as the result of alloy pick-up from the base steel.

- Where higher strength at better ductility is desired, low alloysteel electrodes may be required. Because of greater crack sensitivity of the low alloy steel electrodes, preheating may

be necessary.

- Where corrosion is a factor, it may be advisable to use corewires of the same composition as the base steel.

- Given below are the recommendations for welding sometypical low alloy high strength steels.


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% CompositionElectrodeMaterial

Preheat & inter-passtemperature

(i). C 0.12,Mn 0.5-0.9, Si 0.15

max Ni 0.45-0.75, Cu 0.95-1.30, Al0.12-0.27

Low alloy steel 94 to 260 C

(ii). C 0.12, Mn 0.2-0.5, Si 0.25-0.75, Cr 0.30-1.25, Ni 0.65 max,Cu 0.25-0.55, P0.07-0.015

Low alloy steel 94 to 205 C

(iii). C 0.12 max, Mn 1.25, Si0.10 max, Cu 0.50 max Mild steel Up to 94

C

C 0.40, Mn 0.7-0.9, Si 0.2-0.3 Low alloy steel 205 to 316 C

3. Submerged arc welding

Both hot rolled and heat-treated grades of low alloy steels arewelded by using the method very similar to that used for welding low carbon steels.

Because of deep penetration characteristics of this process, amild steel filler rod usually satisfactory and pre-heating isgenerally not necessary.

4. Thermit Welding

Low alloy high strength steel can be Thermit welded. Metallicelements are added to the thermit mixture to obtain composition close to thata parent metal.

Metallic elements are added either as metallic pieces or in theform of combinations of oxides of the required elements with aluminium.

Stress-relieving heat-treatments, when required, should becarried out between 595 and 675 C.


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5. Resistance spot welding

Spot welding can be carried out satisfactory. For alloyshaving high hardenabilty, special treatments such as pre-heating, grainreinforcement and tempering heat-treatments may be incorporated in thewelding cycle.

6. Other joining process include

- MIG Welding- Atomic Welding- Seam Welding- Brazing.

Reference book: A Text book of welding Technology.O.P.KHANNA

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Alloy Steel Welding