Effect of boron on high-temperature creep behavior of austenitic stainless steel DIN 1.4970
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I.
INTRODUCTION
A M O N G the various factors which influence hightemperature mechanical properties of austenitic stainless steels, the presence of some elements in small quantities and their distribution in the matrix play an important role. Boron is one such element. The presence of this element in austenitic stainless steels is known to improve hightemperature mechanical properties.Ill The effect of boron on the mechanical properties and microstructures of austenitic stainless steels of types 316, 321, and 347 [2] and a niobium-stabilized version of 316 type steel [31 has been studied and reviewed by Thomas and Henry. I4] Though boron is known to improve the mechanical properties of stainless steels, it is an undesirable element when stainless steels are considered for nuclear applications, for boron-10 has a very high (n, a) cross section for thermal neutrons and helium thus produced causes helium embrittlement. [5] The effect of boron distribution on the mechanical behavior has also been studied. Williams and co-workers [6'71 have shown that the nonequilibrium segregation of boron on the grain boundaries can be controlled by adjusting the cooling rates after heat treatments. Boron is also known to be incorporated into M23C 6 precipitates on the grain boundaries, is] In this paper, we report investigations of the influence of boron on the creep life and ductility of a stainless steel designated DIN 1.4970. Prior to creep tests, microstructural characterization was carded out by transmission electron microscopy. Boron distribution before creep tests was mapped using a-autoradiography. Attempts are made to understand the creep data in terms of microstructure and boron distribution in the matrix.
R.V. NANDEDKAR, formerly with the Indira Gandhi Centre for Atomic Research, Kalpakkam, India, is Staff Scientist with the Centre for Advanced Technology, Indore 452012, India. W. KESTERNICH, Staff Scientist, is with the Institut ftir FestkOrperforschung der Kernforschungsanlage and the Association Euratom Kernforschungsanlage, Jtilich D-5170, Federal Republic of Germany. Manuscript submitted March 1, 1990.
METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
Stainless steels which are investigated in this paper are DIN 1.4970, a titanium-stabilized stainless steel, and a nearly boron-free version of it (henceforth, it will be referred to as 1.4970 LB). The chemical compositions of these steels are given in Table I. The boron content of both the steels was determined by spark source mass spectroscopic analysis within 1 ppm accuracy. The steels were cold-rolled, and the following heat treatments were given to both of the steels: 1. solution annealing at 1100 ~ This set of samples will be referred to as SA. 2. solution annealing + 15 pct cold work + aging at 800 ~ for 24 hours. This set of samples will be referred to as standard treatment (ST). All heat treatments were performed in a vacuum of < 10 -5 Pa. Before the final heat treatment, creep specimens having a gage length of 12 m m and width of 2 m m were cut by
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