1.4669, a new lean duplex stainless steel with improved toughness and machinability

Among duplex stainless steels, the lean duplex family is a quite new family, still in expansion. It allows a goodcorrosion resistance, most of the time over that of a standard 4301 austenitic grade, to high mechanicalproperties, far higher than that of a 4301, and to a low amount of expensive alloying elements such as nickelcompared to that of a 4301.But when these grades are produced in high diameter bars, they often present a bad toughness and a poormachinability, these properties being critical when trying to use such high diameter bars in variousapplications. For example, the well-known 4062 and 4162 grades present an impact strength below 100 J at20°C and below 50 J at - 46°C on ?73mm bars. Moreover, their machinability in terms of tool wear and chipbreakability is below that of 4301 grades, especially when these last ones are of an improved machinabilityversion, such as UGIMA®. The poor machinability of these lean duplex grades is mainly due to their highmechanical properties which induce high cutting forces on the tools during a machining operation, thus, rapidtool wear, and is also due to their very low sulphur content (less than 10ppm) which does not help the chipbreaking contrarily to what happens on a 4301 grade with 0,025%S.It is the reason why UGITECH developed these last few years the 1.4669, a new lean duplex grade with a lowernitrogen content and a higher copper content in order to improve the toughness of this kind of grades and tolower the tool wear rates when machining them via a decrease of the cutting forces on the tools. Moreover, acontrol of the inclusions in the grade was performed in order to improve the chip breakability of the gradewhen machined. Of course, this new grade keeps a corrosion resistance over that of a standard 4301.INTRODUCTIONAmong duplex stainless steel, the lean duplex family is a quitenew family, still in expansion. Themost known are the 1.4062 andthe 1.4162 Lean Duplex Stainless Steels (LDSS). Compared to the1.4362, these new LDSS have a lower amount of expensive alloyingelements such as nickel (between 1,5 and 3% compared tothe 4,5% of the 1.4362). To keep a good ratio between ferrite andaustenite (not so far from 50/50) in these two LDSS, their N wasraised from around 0,12% to 0,2% and, in 1.4162, chromium wasslightly decreased (from 22-23% for 1.4362 to 21% for 1.4162).The consequences of these chemical analysismodification aremultipleas detailed in paragraphs 1 to 3: loss in toughness, in machinability(in terms of tool wear rates), and in some cases in corrosionresistance compared to the standard 1.4362. So the questionis: how can we improve the machinability and toughnessof a LDSS, without too expensive alloying elements and keep, atthe same time, a corrosion resistance equal of over that of a 1.4301austenitic SS?TOUGHNESS OF 1.4062 AND 1.4162 LDSSDifferent ? 73mm bars of 1.4062, 1.4162 and 1.4362 were industriallyproduced in order to compare their toughness, corro-Nicolas Renaudot, Eric Chauveau, Marc MantelUGITECH research centre, FrancePaper presented at the 7th European Stainless Steel Conference -Como, 21-23 September 2011sion resistance and machinability. The chemical analysi

Chip formation mechanism and machinability of wrought duplex stainless steel alloys

This paper investigates the chip formation mechanism and machinability of two-phase materials, such as, wrought duplex stainless steel alloys SAF 2205 and SAF 2507. SEM and optical microscopic details of the frozen cutting zone and chips revealed that the harder austenite phase dissipates in the advancement of the cutting tool, being effectively squeezed out of the softer ferrite phase. Microhardness profiles reveal correlation in hardness from the workpiece material transitioning to the chip. The tool wear (TiAIN + TiN coated solid carbide twist drill) and machining forces were investigated. Tool wear, was dominantly due to the adhesion process which developed from built-up edge formation, is highly detrimental to the flank face. Flute damage was also observed as a major issue in the drilling of duplex alloys leading to premature tool failure. Duplex 2507 shows higher sensitivity to cutting speed during machining and strain hardening at higher velocity and less machinability due to presence of higher percentage of Ni, Mo and Cr