Investigation of the pore-formation process during the chromizing of steel 40Kh by the circulation method

Abstract

During the chromium plating of carbon steels, a carbide possessing elevated hardness and wear resistance is formed on the surface. It is known that the development of diffusion porosity, which is associated with the different diffusion mobilities of the atoms, is possible in the coatings, Use of carbides as an anti-corrosionWe investigated steel 40Kh specimens (0.36-0.44% C; 0.8-1.1% Cr; 0.17-0.37% Si; and, 0.5-0.8% Mn), which were fabricated in the form of disks 20 mm in diameter and 5 mm thick. The disks were ground with a minimum cutting depth of 2 #m. The thicknesses of the disks were measured in several sections on a IZV-1 length gage with a scale division of 1 ~m. The spread of the thickness values amounted to + 1 ~m. The specimens were chromium-plated in a DKh2 laboratory circulation unit at 1000°C for 2 and 5 h. The circulation method of diffusion saturation, which is applicable in the present investigation, is promising, since it is possible to develop production regimes to achieve a continuous protective layer [1].After chromium plating, the specimens were subjected to prolonged tempering at 1000°C for 50 and 100 h. To prevent oxidation, they were soldered into a quartz ampule filled with commercially pure Grade "A" argon with no more than 0.003 and 0.1% of O 2 and N 2 impurities, respectively. To exclude possible rupture of the ampule during heating, the argon was evacuated with a residual pressure of 13.3 Pa using a vacutan pump prior to its being soldered. Annealing was carried out in a laboratory furnace with automatic temperature maintenance in accordance with the following scheme: heating to 1000*C for 4 h, holding for 8 h, and cooling to room temperature over 12 h. This regime was repeated several times so that the overall holding time ran n = 50 or 100 h. The temperature was controlled using a TKhA thermoelectric transducer. Some of the specimens were removed from the furnace after being held for 50 h at 1000°C. The remaining specimens were resoldered and subjected to further annealing for a period ran n = 100 h.The annealing temperature of 1000°C corresponded to the chromium-plating temperature and was selected as methodologically convenient for investigation of pore-formation processes during the production of a carbide coating.Metallographic investigations and measurement of the thickness of the carbide layer was carried out under a "Neophot-21" optical microscope. The specimens were etched in Murak [10 kg of K3(FeCN) 6, 10 g of NaOH, and 100 ml of H20] and nital (3-5 % solution of HNO 3 in ethyl alcohol) reagents to expose the structure of the surface carbide layer [2]. Micro-X-rayspectral analysis of the specimens was carried out at the I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy under a BS-340" electron microscope with an X-ray analyzer manufactured by the "Link Analytical" Co. The visible porosity on three segments of microsections under a magnification of x 1000 was assessed on the analyzer via a special program, the overall field of view was 170/zm2; this ma...

Corrosion resistance of chromized 316L stainless steel for PEMFC bipolar plates

“…Considering the above-mentioned requirements, the pack cementation technique appears to be an attractive surface treatment to improve corrosion resistance without increasing electrical resistance. The pack cementation technique has been employed extensively to improve corrosion resistance or increasing the surface hardness and the experimental methods and the diffusion mechanism during cementation process is well established [18][19][20][21][22][23].…”