Cracks- hot cracking-cold cracking-base metal cracking, Mechanical Engineering

Cracks

 

Cracks are the linear ruptures of the metal under stress. Sometimes they appear large and frequently they are narrow separations. The major classification of cracks is:

  • Hot cracking
  • Cold cracking
  • Micro fissuring
  • Base metal cracking
  • Crater cracking.

 


Hot cracking: Hot cracking occurs at elevated temperature during cooling and solidifying from the molten stage. The main causes are the restraint on the joint and the presence of higher carbon and sulphur. The other reason is segregation of weld metal due to bead shape. Hot cracks are inter-granular. For avoiding hot cracks, it is essential to keep these joints with least restraint and to have least amount of heat input. Proper matching of electrodes and cooling rate will reduce the probabilities of cracking. The cracks on the craters of weld metal are examples for hot cracking. MPI or LPI are used to detect surface cracks while ultrasonic testing is suitable for internal cracks.

Cold cracking: Cold cracking in steels refers to cracking which occurs even after a few days. In general, cold cracking starts in the Heat Affected Zone (HAZ) of the weld metal. This is primarily associated with combined effects of hydrogen, restraint & martensite formation. Increasing the carbon content in the base metal andthe manganese content in the weld metal also prompts cold cracking. To minimise cracking it is essential to use low hydrogen electrodes and pre heating. Usually MPI for ferrous materials and LPI for non ferrousmaterials are used to detect these cracks at surface.

Micro fissuring: These are very small fissures detectable only at high magnifications. They are caused by hot or cold cracking.

Base metal cracking: Cracking immediately along the weld or brittle fusion zone especially in high carbon or alloy steels areusually hardening cracks. These are the results of too high a cooling rate, leading to martensite formation in the HAZ. They are also called "underbead cracking". Since ductility usually decreases with increasing hardness, base metal cracking is associated with lack of ductility in the HAZ. MPI or LPI are the best suited to detect these types of cracks. Inclusions and laminations present in the base metal are usual reasons for base metal cracking. The factors increasing cracking tendency are

  • Increasing wall thickness
  • Increasing restraint
  • High strength of base or filler metal
  •  Easy hardenablity of base or filler metal

 

Posted Date: 9/18/2012 12:34:27 AM | Location : United States







Related Discussions:- Cracks- hot cracking-cold cracking-base metal cracking, Assignment Help, Ask Question on Cracks- hot cracking-cold cracking-base metal cracking, Get Answer, Expert's Help, Cracks- hot cracking-cold cracking-base metal cracking Discussions

Write discussion on Cracks- hot cracking-cold cracking-base metal cracking
Your posts are moderated
Related Questions
Define the process of SAW (submerged Arc welding). And also explain its advantages and disadvantages.

Lean Mixture: It generally ranges from approximately 15 : 1 to 17 : 1. By the use of this fuel, engine runs for higher economy.

T ype s of Loading: Mainly three types of load acts on any beam; 1. Concentrated load ( or point load) 2. Uniformly distributed load 3. Uniformly varying load

Belt tension for V-belt: What is V-belt. Drive expression of Ratio in belt tension for V-belt Sol: The power from one shaft to the other shaft can be transmitted with t

A rotor has the given properties : If the shaft is balanced by two counter masses located at 100 mm radii and revolving in planes midway of plan 1 and 2 and midway of 3 and

There is risk of wet H 2 S corrosion in all emulsion and gas lines from the Inlet Separator up to and including the Treater. Full compliance to NACE is highly recommended to contro


Function of heat pump: Refrigerator The primary function of heat pump is to transfer heat from the low temperature system to the high temperature system, this transfer of


The cross-section of two angles attached together are shown on the left below. The properties of angles are given in the table with reference to the diagram on the right. The ma