CSWIP 3.1: Question with Answer and Explanation – Part 16

CSWIP 3.1 QUESTION AND ANSWER SERIES

1) Which of the following processes is most prone to solidification cracks?

a.	MMA with basic coated electrodes
b.	MMA with rutile coated electrodes
c.	TIG
d. ( answer )	SAW

Solidification also referred as Hot Cracking.

Solidification Cracking (hot crack) can occur when:

-Weld metal has a high carbon or impurity (sulphur) content

-The depth-to-width ratio of the solidifying weld bead is large (deep & narrow)

-Disruption of the heat flow condition occurs, stop/start condition

2) Which of the following could be used to minimise the occurrence of solidification cracks in steels welds?

a.	Increase sulphur levels to a minimum of 0.5%
b. ( answer )	Reduce the dilution into the base metal
c.	Increase carbon content in the weld metal
d.	Reduce the manganese content in the weld metal

3) Hydrogen cracking is considered a cold crack as it will not form till the weldment cools to below:

a.	500oC
b. ( answer )	300oC
c.	300oF
d.	730oC

300oF = 149oC

400oF = 205oC

HYDROGEN INDUCED CRACKS: As know cold cracking, delayed cracking or underbead/toe cracking.

4) In a heavy plate fabrication where S is residual @0.3%, which may cause problems in the welded joint, additions of Mn may be added to prevent:

a.	Laminations in the plate through thickness
b.	Stress corrosion cracks
c.	Manganese sulphides (MnS)
d. ( answer )	Solidification cracking

Solidification Cracking (hot crack) can occur when: Weld metal has a high carbon or impurity (sulphur) content.

Precautions for controlling solidification cracking:

Add Manganese to the electrode to form spherical Mn/S which form between the grain and maintain grain cohesion.

As carbon increases the Mn/S ratio required increases exponentially and is a major factor. Carbon content % should be a minimised by careful control in electrode and dilution.

5) Inspection for hydrogen cracking is often specified to be done between 48 to 72 hrs after completion of the weld, this is because?

a.	The stress level will have reduced by then
b. ( answer )	It is the maximum time it taken for all the H2 to diffuse out of the weld
c.	Usually after 48 hrs such cracking is unlikely to occur
d.	It is to ensure the weld has property cooled down to ambient temperature

Typically, hydrogen-induced cracking occurs at temperatures below 600 degrees Fahrenheit (more commonly around or below 300 degrees Fahrenheit) and within 48 hours of completing the weld. In some cases, it can occur as much a week after welding.

In applications with greater contributing factors — highly restrained joints; high levels of hydrogen; or rapid cooling after weld completion — hydrogen-induced cracking can occur much sooner, even immediately.

6) Preheats are used on steel joints of high hardenability mainly to:

a. ( answer )	Slow the cooling rate of the steel
b.	Remove surface moisture from the joint
c.	Reduce the formation of surface oxides
d.	Increase the diffusion rate of hydrogen into the HAZ

There are four primary reasons to utilize preheat:

(1) it lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking

(2) the slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking

(3) it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints and

(4) it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as notch toughness.

7) Sensitisation is a term applied to the formation of which intermetallic compound at the grain boundaries in the HAZ of austenitic stainless steels?

a. ( answer )	Chromium carbide
b.	Titanium carbide
c.	Niobium carbide
d.	Molybdenum carbide

Crack type: Inter-granular corrosion

Location: Weld HAZ. (longitudinal)

Steel types: Stainless steels

Microstructure: Sensitised grain boundaries*

Occurs when:

An area in the HAZ has been sensitised by the formation of chromium carbides.This area is in the form of a line running parallel to and on both sides of the weld. This depletion of chromium will leave the effected grains low in chromium oxide which is what produces the corrosion resisting effect of stainless steels. If left untreated corrosion and failure will be rapid*

8) When comparing the properties of plain carbon steels and austenitic stainless steels … which of the following statements is true?

a. ( answer )	Carbon steels have better thermal conductivity
b.	Stainless steels have low expansion and contraction
c.	Carbon steels have higher expansion rates in the Z direction
d.	Stainless steels have lower chromium

Austenitic stainless steels have high ductility, low yield stress and relatively high ultimate tensile strength, when compare to a typical carbon steel.

Plain Carbon Steels: