Masters Theses

Date of Award

12-1985

Degree Type

Thesis

Degree Name

Master of Science

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Abstract

The primary purpose of the present research was to investigate the susceptibility to hot cracking of 316 NG, modified 347 and 304 NG stainless steels which have been approved by the Nuclear Regulatory Commission for BWR use. In addition, the hot ductility of these steels was also determined and correlated with the base metal HAZ hot cracking tendency.

A new multi-bead varestraint technique was developed which provides simultaneous evaluation of the base metal HAZ, weld metal HAZ and the fusion zone of the as-deposited pass. The hot ductility test was conducted on the Gleeble using standard parameters.

Both, the varestraint test and Gleeble test were utilized to evaluate the hot cracking susceptibility of base metal HAZ in austenitic stainless steels. In addition, the fusion zone and weld metal HAZ susceptibility to hot cracking was determined with the varestraint device. Optical metallographic, SEM fractography and energy dispersive X-ray analisis were employed to define the cause of hot cracking in the austenitic stainless steels.

The fusion zone hot cracking susceptibility behavior is in line with currently applied concepts related with delta ferrite content and primary ferrite solidification. The 347 materials are less crack sensitive in the fusion zone than the low ferrite (0-1.5 FN) 316 NG materials. Japanese 347 LPN and German TP 347 Nb exhibit the best behavior with the French ICL 473 Nb and AISI grades being slightly more crack sensitive, as far as susceptibility to cracking in the fusion zone is concerned.

The results of the weld metal HAZ cracking sensitivity after exposure to 3 passes shows the 347 LPN and the TP 347 to be more resistant to weld metal HAZ cracking than the AISI and ICL 473 Nb grades. The 316 NG and 347 materials behave similarly and can be expected to respond in a like maner in an actual weld situation. Additional work is needed in this area of investigation to define the significance of the results and the relationship to the proposed theory of Lundin and Chou (3).

As regards base metal HAZ cracking sensitivity, a separation of the materials can be made on the basis of the total crack length and threshold strain; the AISI 347 material, among the 347 types tested and the 316 NG heat D-441103 are the most sensitive to base metal HAZ cracking.

The advantages of the varestraint test device are: economical preparation of specimens, efficient testing, sensitivity to small changes in a test variable, reproducibility of results free of variation due to human factors, to universal application to all welding processes.

In hot ductility tests, all the 316 NG materials exhibited HI behavior on heating. However, for these materials, the on-cooling behavior shows heat to heat variations. One heat (D441103) of the five Sumitomo heats demostrates a class C2 on-cooling behavior, showing a ductility dip at approximately 1800°F on-cooling from the ZDT of 2475°F.

AISI 347 among the 347 stainless steels shows the poorest recovery on hot ductility on-cooling (C3) from ZDT of 2450°F, virtually no recovery in ductility until the on-cooling test temperature reach 2200°F. These behavior is in concordance with the varestraint test results on base metal HAZ.

In hot ductility testing when the peak temperature of the test is lower than the zero ductility temperature (ZDT), the recovery of on-cooling ductility is better than on-cooling from the ZDT. The Gleeble hot ductility test should be considered in light of the following restrictions for evaluation of hot cracking of welds:

a) The information gained from the Gleeble applies only to base metal HAZ as the fusion zone or the multipass weld metal HAZ is not evaluated.

b) If the ZDT is not precisely determined (+- 25°F) the material can show variable response in hot ductility.

The metallographic examination of the hot ductility tested 316 NG and 347 materials reveals that the intergranular fracture at the ZDT is related to the grain boundary liquation, there are two types of secondary cracks; first one is along the grain boundary which is roughly parallel to the fracture surface, second one is along the preferentially melted segregation bands. In AISI 347, the second type of secondary cracking is due to the constitutional liquation in the niobium carbide containing band during the on-heating thermal portion of the cycle to ZDT.

Metallographic and fractographic examination in the \Nfeld metal HAZ and base metal HAZ shows that the cracks are found only at the solute enriched grain boundaries adjacent to the fusion line. In 316 NG the grain boundaries were found to be enriched in silicon and molybdenum. In the 347 materials the cracks were found at the solute rich grain boundaries with enriched silicon and liquid films produced by constitutional liquation associated with niobium carbide. Comparison between the varestraint test and the Gleeble test shows that the varestraint test appears to be more sensitive than hot ductility test in assessing the hot cracking susceptibility of the materials. In addition the varestraint device is less expensive than the Gleeble and it is easier to obtain data of a wide applicability.

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