Brush up your knowledge

A) What role does surface active elements play in A-TIG welding process?

  1. Decreases temperature coefficient of surface tension
  2. Increases temperature coefficient of surface tension
  3. None of the above

B) The minimum critical factors for hydrogen cracking could be which one of the following?

  1. Hydrogen content more than 15 ml per 100 gm weld deposit
  2. Hardness more than 400 VPN
  3. Temperature less than 350 deg. C
  4. All of the above

C) Solidification cracking is caused generally by which condition?

  1. High carbon content
  2. High sulfur content
  3. High quenching of steel
  4. High hydrogen content

D) Surface tension between the solid grain and the grain boundary liquid shall be _____ to avoid solidification cracking.

  1. Low
  2. High
  3. None

E) Which process has maximum Arc efficiency among below mentioned processes?

  1. Submerged arc welding
  2. Gas tungsten arc welding
  3. Shielded metal arc welding
  4. Gas metal arc welding

F) Tendency of arc blow will be _____ for same polarity electrodes having distance less than critical value.

  1. Outside
  2. Inside
  3. No arc blow

G) Effect of low inductance in GMAW process- short circuit mode?

  1. Smooth flow of molten metal
  2. More spatter
  3. Welding gun is pushed back
  4. None of the above

H) We know higher velocity of moving heat source leads to higher centerline segregation, this will _______ the toughness of the weld joint.

  1. decrease
  2. increase

I) Practically upto how much concentricity in Stick electrodes will not create any problem?

  1. 2 thou
  2. 6 thou
  3. 4 thou
  4. 8 thou

J) A quantitative means for assessing the relative degree of protection v/s killing in a particular process or welding consumable is the ________.

  1. Oxygen scale
  2. Hydrogen scale
  3. Nitrogen scale
  4. None of the above

Keep reading, Happy welding

Thank you,

KP Bhatt

Answers:

A) 2,

B) 4,

C) 2,

D) 2,

E) 1,

F) 2,

G) 2,

H) 1,

I) 3,

J) 3.

Methods to minimize hydrogen cracking in HAZ (heat affected zone)

As discussed in previous article, activation of four factors simultaneously influences hydrogen cracking in HAZ. Methods to minimize influence of each factors is described here:

  1. Hydrogen

The main source of hydrogen is moisture (H2O) and principle source of moisture is welding flux. Some fluxes contain cellulose and this can be a very active source of hydrogen.

Reducing the influence of hydrogen is possible by:

  • Ensuring that fluxes are low in H when welding commences.
  • Low hydrogen electrodes must be either baked and then stored in a hot holding oven or supplied in a vacuum sealed packs
  • Basic agglomerated SAW fluxes should be kept in a heated silo before issued for welding
  • Checking the amount of moisture present in the shielding gas by checking the dew point(must be below -60oC)
  • Ensuring the weld zone in dry and free from rust/scale and oil/grease
  1. Tensile stress

There are always tensile stresses acting on a weld because there are always residual stresses from welding. The magnitude of tensile stresses is mainly dependent on the thickness of the steel at joint, heat input, joint type and the size and weight of the components being welded.

Practical ways of reducing the influences of residual stresses may be:

  • Avoiding stress concentrations due to poor fit up.
  • Avoiding poor weld profile
  • Increasing the travel speed to reduce heat input
  • Keeping weld metal volume as low as possible
  1. Susceptible HAZ microstructure

A Susceptible HAZ microstructure is one that contains a relatively high proportion of hard brittle phases of steel, particularly martensite.

Avoiding a susceptible microstructure requires:

  • Using moderate welding heat input so that the weld does not cool quickly
  • Apply pre-heat so that the HAZ cools more slowly; in multi run welds, maintain a specific interpass temperature
  1. Weldment at low temperature

Weldment temperature has a major influence on susceptibility to cracking mainly by influencing the rate at which H can move through weld and HAZ.

Reducing the influence of low weldment temperature can be effected by:

  • Applying a suitable preheat temperature(typically 50 deg. C to 250 deg. C)
  • Maintaining preheat temperature and interpass temperature
  • Hold preheat temperature after completion of welding for atleast 2 hours so that H gets time to escape from solid metal
  • Post-heat(De hydrogenation treatment)

Reference: TWI literature

Keep reading, Happy welding

Thank you,

KP Bhatt