Influence of grinding operations on surface integrity and chloride induced stress corrosion cracking of stainless steels

Stainless steels were developed in the early 20th century and are used where both the mechanical properties of steels and corrosion resistance are required. There is continuous research to allow stainless steel components to be produced in a more economical way and be used in more harsh environments. A necessary component in this effort is to correlate the service performance with the production processes. The central theme of this thesis is the mechanical grinding process.  This is commonly used for producing stainless steel components, and results in varied surface properties that will strongly affect their service life. The influence of grinding parameters including abrasive grit size, machine power and grinding lubricant were studied for 304L austenitic stainless steel (Paper II) and 2304 duplex stainless steel (Paper I). Surface integrity was proved to vary significantly with different grinding parameters. Abrasive grit size was found to have the largest influence. Surface defects (deep grooves, smearing, adhesive/cold welding chips and indentations), a highly deformed surface layer up to a few microns in thickness and the generation of high level tensile residual stresses in the surface layer along the grinding direction were observed as the main types of damage when grinding stainless steels. A large degree of residual stress anisotropy is interpreted as being due to mechanical effects dominating over thermal effects. The effect of grinding on stress corrosion cracking behaviour of 304L austenitic stainless steel in a chloride environment was also investigated (Paper III). Depending on the surface conditions, the actual loading by four-point bend was found to deviate from the calculated value using the formula according to ASTM G39 by different amounts. Grinding-induced surface tensile residual stress was suggested as the main factor to cause micro-cracks initiation on the ground surfaces. Grinding along the loading direction was proved to increase the susceptibility to chloride-induced SCC, while grinding perpendicular to the loading direction improved SCC resistance. The knowledge obtained from this work can provide a reference for choosing appropriate grinding parameters when fabricating stainless steel components; and can also be used to help understanding the failure mechanism of ground stainless steel components during service.

Flerskiktat papper : en sammanställning av historia, teknik och forskningsresultat

Until the beginning of the 1900:th century the paper making process was handicraft. The paper machines that have been developed since then are as impressing in size as fascinating when it comes to the technique. The process has always been improved to give a better paper for lower costs, with as short manufacture time as possible. Stratified forming has been a reality since 1830. From the beginning it was applied to board and paperboard. Now it is also the most common method for making tissue and one paper mill in Austria use it for fine paper. Stratified forming can be categorized as separate forming or simultaneous forming. Separate forming means using several headboxes and sometimes even several wires. Simultaneous forming means using only one stratified headbox. This method has many advantages over separate forming, inter alia improved economy, quality, wood exchange and higher runability of the paper machine. Several experiments have been done with simultaneous forming of different fibres by placing a fibre with high bulk in the middle layer and a smooth fibre on the surface layers. The results has shown that by using this method both the bending stiffness and surface properties are improved, or a lower grammage paper is obtained with maintaining quality. Simultaneous forming can also be used to stratify fillers in layers where they are most efficient. In that way both paper quality and economy is improved. Simultaneous forming is also used for fractionated pulp. Fractionation means separating springwood fibre from summerwood fibre, and placing them in separate layers. Research results of fractionated pulp show about the same result as simultaneous forming of different fibres.