Tuesday, August 18, 2009

Controlled Thermal Severity Test

Armour steels are generally low alloy steels which are hardenable. The steel develops high strength and high hardness with adequate fracture toughness. Hence a fine balance should be maintained in the chemistry of the steel and its heat treatment. As told earlier the steel had passed all the ballistic tests and what was required to be proved was that the steel can be welded.

High hardness steels are susceptible for what is called underbead cracking. These cracks are buried cracks which develop under the weld bead and extremely dangerous since they are not detected after welding and generally take some time to develop. Three conditions are essential for this type of cracks to develop.
1. There must be adequate hydrogen in the weld. This hydrogen comes from the coating on the electrode or from some external agent such as grease or oil used for preserving the steel.
2. Tensile stresses must be present in the welded structure.
3. There should be susceptible microstructure, that is , microstructure such as martensite.

Since we had no earlier knowledge of welding armour steel we did a literature survey. We could not get much information, naturally, as the welding of armour steel was a classified information. We decided to start from fundamentals. All high hardness steels are tested for underbead cracking by a test called Controlled Thermal Severity Test. We decided to apply this test to the armour steel with some modification which was permitted for acceptance of the results obtained. The tests are to be done in a controlled fashion which means that the test conditions have to be standardised. We developed a gadget which allowed us to measure the heat input precisely. How we performed the tests and how we interpreted the tests will be explained later.

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