EMBRITTLEMENT OF HIGH-STRENGTH STEELS : EFFECTS OF IMPURITIES AND HYDROGEN.

Format:

Book

Thesis/Dissertation

Author/Creator:

BANDYOPADHYAY, NIKHILES.

Contributor:

University of Pennsylvania.

Subjects (All):

Metallurgy.

0743.

Local Subjects:

0743.

Physical Description:

241 pages

Contained In:

Dissertation Abstracts International 41-10B.

System Details:

Mode of access: World Wide Web.

text file

Summary:

A study has been made of the fracture behavior at various test temperatures of laboratory and commercial heats of 4340-type steels, and of a plain-carbon steel, all quenched to form martensite and tempered at various temperatures below 525(DEGREES)C. The toughness trough associated with tempered martensite embrittlement (TME) was observed in the commercial steels, but was absent in a "pure" laboratory heat free of Mn and Si. However, tests at 77K, showed a trough even for the "pure" heat and the trough was found to be associated with about 20% intergranular fracture. Room temperature four-point bend tests on pure and commercial steels showed behavior similar to the Charpy tests. The notch root strain for room temperature fracture initiation showed troughs for both the commercial heat and the pure heat with Mn + Si near the 350(DEGREES)C tempering temperature, but the trough was once again absent for the pure heat. It was found that the fracture was initiated by rupture and switched to partly intergranular just ahead of the initial rupture. However, when the test were done at 77K, the critical local fracture stress ((sigma)*) showed TME troughs for all three steels. The troughs were again associated with a considerable amount of intergranular fracture. By scanning Auger analysis, segregation of both sulfur and phosphorus was detected on the prior austenite grain boundaries in these steels. Therefore, these results suggest that TME is mainly due to grain boundary weakening by segregated sulfur and phosphorus, and it is triggered by carbide formation at tempering temperatures above 150(DEGREES)C. Carbide formation is a necessary, but not sufficient, condition for this type of embrittlement.

The behavior of these steels in a gaseous hydrogen atmosphere was found to be extremely sensitive to small changes in composition, yield strength, and hydrogen fugacity or pressure. In order to get a clear picture of these effects, experiments were done in hydrogen atmosphere and also in the absence of hydrogen. Comparing K(,IC) in air and K(,TH) for cracking in hydrogen against a composition parameter (Mn + 0.5Si + S + P), it was found that at low values of the parameter, hydrogen had little effect on K(,TH), although it did produce a change in fracture mode from rupture to interface decohesion. As the parameter increased, however, K(,TH) dropped well below K(,IC). This corresponds to an increase in the amount of cracking along prior austenite grain boundaries. Therefore, it is believed that the principal effect of the composition parameter is to control the amount of P and S segregated to the grain boundaries during austenitization, and thus the strength of these boundaries.

It is postulated that the cohesion of steel can be lowered by an increase in concentration of either hydrogen or metalloid impurities. Hydrogen segregates to regions of hydrostatic tension even at (TURN)room temperature (because of its extra ordinary high mobility and large atomic volume in the bcc Fe), whereas surface-active impurities segregate to grain boundaries only at high temperature. The value of local hydrogen concentration C(,H) at equilibrium can be calculated from the thermodynamic expression given by Li et al.

(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)

where C(,0) is the equilibrium hydrogen concentration in the unstressed lattice and according to Sievert's law, C(,0)(alpha)p(,H(,2))(' 1/2), (theta) is the hydrostatic tension. In principle C(,H) can be calculated as a function of yield strength of the material by means of an elastic-plastic stress analysis. For a given hydrogen fugacity, the strength of the material that can be safely used is shown to depend upon the composition, and therefore strength, of the grain boundaries.

Notes:

Source: Dissertation Abstracts International, Volume: 41-10, Section: B, page: 3859.

Thesis (Ph.D.)--University of Pennsylvania, 1980.

Local Notes:

School code: 0175.

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