Tuesday, August 25, 2009

Meeting at DMRL

There was a high level meeting in DMRL which was attended by all big bosses of ordnance factory board (OFB). The Director of DMRL and his associates made a detailed presentation of the development and the various tests conducted by them. Discussions went on and on. The questions asked by the OFB board members were answered patiently by DMRL staff even though many questions were flimsy in nature. Then it was my turn to present the experiments done and interpretations of the results obtained. I had already mentioned that OFB's were way behind in their standards for armour testing. Hence the method of testing adopted by us was challenged. I presented enough data and informed the committee that the testing procedures adopted by us was in line with the British armour testing specifications. This seems to have convinced them that we were on the right track. In addition to the CTS test we had welded one meter long plates and after conducting radiography we carried out conventional tests such as tensile test, bend test and charpy impact test at low temperature. All tests were in acceptable limits. One of the persons who was very sarcastic during the review suddenly told me that welding one meter does not prove that the steel is weldable as the armoured personnel carrier has hundreds of meters of weld. I retorted by saying that in my company we just weld a test coupon which is 450 mm long and if the test coupon passes all the tests based upon that procedure we weld thousands of meters. He did not like my answer because no body has ever talked to him like that. Later I learnt that he was one of the top people in the army. He was not accustomed to a civilian snubbing him. In spite of DMRL clearing all doubts OFB insisted that DMRL should weld a full scale hull of the APC, fit the engine and the transmission, carry all sorts of tests and if the tets are satisfactory then only army will agree for the Jackal steel to be used as a replacement steel for APC. DMRL requested BHPV to manufacture one hull with the drawings supplied by them. We did produce a hull which was fitted with the engine and transmission. This prototype APC went through all trials and was finally accepted by the army. During these trials not a single weld failed certifying that the technology developed by us met all the requirements of OFB.The welding technology supplied by us is even now used for welding indigenous armour plate developed by DMRL.
In the end we told DMRL we can weld armour steel!

Wednesday, August 19, 2009

CTC Test

Controlled thermal severity test is very sensitive test to evaluate a steel for its propensity for underbead cracking. This test is in use for a number of years. We used a modified CTS test developed by The Welding Institute, UK, which was more severe than the classical test.

Controlled therma severity test is done using plates which have propensity for hydrogen induced cracking, also called as underbead cracking. The test spcimen consists of a bottom plate and a top plate which are bolted together. The bottom plate has the dimensions of 175x175 mm x thickness of plate under test. The top plate has the dimension 75x75 mm ,thickness being same as the bottom plate. Both the plates are bolted together securely.Two fillet welds, called anchor welds,are laid between the top plate and the bottom plate.The bolt is again tightened. The test plate is allowed to cool to the room temperature. Using the selected electrode test welds on the remaining sides of the assembly were laid taking care to see that test specimen is not over heated.The welded assmbly is stored for 72 hour for any cracks to develop. After 72 hours the test welds are cross sectioned and polished to see if there any cracks in the heat affected zone. The cross sectioned pieces are subjected to magnetic particle testing to reveal any hidden cracks. Finally the specimen is polished, etched and examined under microscope at 200x magnification to detect any microcracks in the heat affected zone.During testing we had recorded the heat input for which we had developed a gadget. We conducted several tests with various heat inputs.

We selected three types stainless steel electrodes available in the market from three different manufacturers. Initial tests suggested that the standard electrodes which were supplied off the shelf were not meeting our requirements. We called all the electrode manufacturers and requested them to slightly modify the weld chemistry. Only one electrode manufacturer agreed for our proposal and he cooperated with us. Of the three specifications of electrode tested by us one specification fully met our requirements. This electrode was finally recommended to DMRL for welding Jackal steel.

When DMRL presented the data before the ordnance board several questions were raised. Ordnance board followed a method of testing which is no longer in vouge. Board members insisted that we conduct the test to meet their test procedure. Even though their method of testing is outdated to satisfy their ego and bail out DMRL we again did several test as per the standards of ordnance board. Ordnance board fixed another date for final evaluation and I was invited to be present to clarify doubts to ordnance board officials.

Weldability of Jackal Steel

I had talked about armour steel welding.While we were doing literature survey we found that all western countries were using stainless steel electrodes where as Russians were welding with carbon and low alloy steel electrodes.These are classified as ferritic electrode The argument given by the western countries were since armour steels are susceptible for hydrogen induced cracking they decided to use austenitic electrodes for welding.

This needs to be explained. While welding armour steel slightest amount of hydrogen in the weld causes under bead cracking. This hydrogen come from various sources, such as, improperly dried electrodes, rust and grime on the plate, paint marks or condensed water etc.When the plate is welded with a ferritic electrode the hydrogen is first absorbed in the weld metal and as the weld metal cools hydrogen migrates to heat affected zone which contains martensite and in course of time a crack develops. The hydrogen induced cracks take at least 72 hours to develop.

The solubility of hydrogen in the austenitic weld metal is high and is not liberated to heat affected zone after cooling as in the case of ferritic weld metal. Since the migration of hydrogen to heat affected zone is absent the chances of hydrogen induced cracking when armour steel is welded with austenitic stainless steel electrodes are less.

The question arises then why Russians weld with ferritic electrode. They are not dumb. The Russians have a valid argument.

In the manufacture of steel Russians follow a technology of steel making which contains very less hydrogen. More over all their armored vehicles are welded by MIG welding which is known to contain less Hydrogen. More over they used to manufacture large number of armoured vehicles for supplying to their allies almost free. Hence they used ferritic weld metal which is cheaper.

We put all these facts before DMRL and DMRL suggested that we should use austenitic electrodes. Hence we did all our experiments with austenitic elrctrodes,

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.

Monday, August 17, 2009

Can you develop technology for welding armour steel?

We received a message from the Director of Defense Metallurgical Research Laboratory(DMRL), Hyderabad, that he wanted to visit BHPV and in particular R&D laboratory. On the appointed day he visited BHPV and R&D. The first question he asked us was whether we knew how to weld armour steel. Our honest answer was that we had no need to weld armour steel in our business but definitely we can try.

DMRL had developed an armour steel as a replacement for the steel India was importing from Russia for the armoured personnel carrier(APC) which India was building under license.
It was am excellent steel much better than the Russian steel in its ballistic performance. When DMRL proposed to Army to use this steel for armoured personnel carrier (APC) Army had its own reservations.DMRL had manufactured small size plates in its own facilities starting from melting, forging and heat treatment for its trials . The development was remarkable considering the constraint DMRL had. Very few people could understand the manufacture of armour steel and the precise heat treatment it has to undergo to meet the ballistic requirements.In addition, APC will have various thicknesses and the ballistic performance must satisfy various requirements.Credit should be given to the Director, DMRL and his band of enthusiastic young scientists who pulled out a difficult task successfully. You have to remember that this development was in early 1980's. Army, rightly, told DMRL that they need large size plates and in huge quantities. DMRL tied up with one of the steel manufacturers to produce these plates. Once every objection by the Army was neutralised,Army asked DMRL to prove that the steel is weldable and DMRL should supply the welding technology.This is where we came in. We shall discuss next time how we proved that the steel is weldable and the welding technology needed to weld APC's.

Saturday, August 15, 2009

Repairing LOX and LN2 tanks

Stress corrosion cracks can be identified easily. The cracks appear as branches of a tree, that is, there is stem and from the stem branches radiate. When we entered the tank we noticed a number of stress corrosion cracks at the sites where the inner tank was anchored to the outer tank. We did dye penetrant test to reveal the cracks. Some times the cracks are buried inside the material. One way of making the cracks come to the surface is to heat the metal which causes internal cracks to break out to the surface.After ensuring that there are no sub surface cracks, which were revealed by dye penetrant tests, repair work was started. The cracked portion was marked and grinding was done till the cracks were removed. Then the ground portion was welded and checked by dye penetrant test. This method was continued till all the cracks were removed. Then hydraulic test was done and the tank was handed over to the customer.

All five tanks which had developed stress corrosion cracks were successfully repaired and handed over to the customer after hydraulic test. These tanks are performing successfully for the past 15 years.

The repairing of all the tanks took nearly six months not 15 days as expected by MD. If we had not detected the cracks and had done the hydraulic testing all the tanks would have collapsed causing a huge loss to the company. But our MD never realised the gravity of the problem because he was ignorant nor did he appreciate the work done by us. Had we worked in a private company we would have been rewarded by the management. We did not get even one word of appreciation by the management. Such are the motivation people have while working in PSU's!

Sunday, August 2, 2009

What is stress corrosion cracking

Most of the metals are prone to stress corrosion cracking.Stainless steel is more prone to stress corrosion cracking compared to other metals and alloys.Stress corrosion cracks are generally subsurface cracks which are not detected early enough to take corrective measure.
Three conditions are essential for the stress corrosion cracks to occur:
1.The metal temperature must be high, above 70 deg celsius.
2.Tensile stresses must be present in the structure.
3.The atmosphere in which the structure is working must be salty.
Let us look at the LOX and LN2 tanks erected by us for the shore based steel plant.
In the mornings when the atmospheric temperature was around 40 deg.C the metal temperature can be as high as 140 deg.C.
Where the outer stainless steel wall was anchored to the inner carbon steel wall by wedges with fillet welds the stress distribution in the fillet calculated showed high tensile stresses at the stainless steel wall,
Since the tank was erected near the sea moist air saturated with salt condensed inside the tank when the atmospheric temperature fell in the night and deposited salt especially at the fillet joints.In course of time the concentration of salts reached sufficiently high to initiate cracking.These are the cracks we discovered on the steel surface.Stress corrosion cracks generally are buried but in the inner tank where fillet weld was made the cracks were extensive and also cracks can be seen on the inner surface of the tank.How we repaired forms the subject of next blog.