Jyrki Romu

Effects of processing and manufacturing of high nitrogen-containing stainless steels on their mechanical, corrosion and wear properties

Abstract The effects of N alloying through different processing and manufacturing routes on metallurgical, mechanical, corrosion and wear properties of high N-containing stainless steels have been studied. Nitrogen alloying has a well defined and marked improving effect on mechanical and corrosion properties of these steels until the solubility limit of N is reached and nitride precipitation exhibits its deleterious effects on these properties. It is also demonstrated that N alloying contributes significantly to sliding wear and cavitation erosion resistance of stainless steels. The processing and manufacturing technologies of high N-containing stainless steels are evaluated with main emphasis in P/M fabrication including solid-state nitriding of the metal powder.

Grain refinement processes for superplastic forming of AISI 304 and 304L austenitic stainless steels

Abstract Elongation to fracture values higher than 300% were obtained for cold rolled AISI 304 type steel grades in hot tensile tests. The effects of heating rate and carbon content on the elongation of the steels studied are discussed based on internal friction measurements and TEM observations. A slow heating rate to the test temperature and high carbon content of the steel led to a fine and equiaxial grain structure of the steels.

Manufacturing of Shaped Forms from Stainless Steels with Superplastic Forming

Shaped forms have been manufactured with superplastic forming from austenitic (EN 1.4318) and microduplex stainless steels (EN 1.4362). The aim was to find suitable process parameters for manufacturing of the prototypes. Based on the results of hot tensile tests and cone cup tests, the forming temperatures used were 800 and 960 °C for EN 1.4318 and 960 °C for EN 1.4362. The possibility to lower forming temperature by using EN 1.4318 may offer savings in manufacturing costs. However, longer forming times are needed for EN 1.4318 than for EN 1.4362, which increase the process costs for EN 1.4318. Alltogether, further developments in the thermomechanical processing of EN 1.4318 are needed in order to make it more competitive as compared with duplex stainless steels.

Effect of Electrochemical Potential on Stress Corrosion Cracking Susceptibility of EN 1.4301 (AISI 304) Austenitic Stainless Steels in Simulated Hot Black Liquor

Stress corrosion cracking (SCC) susceptibility of austenitic EN 1.4301 (AISI 304, UNS S30400) stainless steel was studied as a function of electrode potential at T = 190°C in 15 g/kg NaOH + 150 g/kg Na2S containing caustic environment simulating heavy black liquors (HBL) of the pulp industry. Severe cracking was detected at the corrosion potential and at the cathodic potential of −0.11 VMo/MoS2 reference electrode. On the other hand, at anodic potentials of 0.03 VMo/MoS2 to 0.3 V Mo/MoS2 no cracking was observed. Thus, SCC of EN 1.4301 steel can potentially be mitigated in HBL environment by applying anodic protection. At the corrosion potential, selective dissolution of Fe and slight localized enrichment of Ni and Cr, as well as Na, S, and O, was observed. At anodic potentials, Fe was selectively dissolved and marked enrichment of both Ni and Cr, as well as Na, S, and O, took place in the corrosion product. The simultaneous enrichment of Ni and Cr in the corrosion product film was concluded to be the pre...

Metallurgical response of weld metal to different filler metal and joint design combinations of laser-arc hybrid welded lean duplex and novel ferritic stainless steels

Laser and laser-arc hybrid welding of duplex and ferritic stainless steels is demanding, because microstructure of the welds tends to be highly ferritic. Therefore, filler metal must be used for maintaining corrosion and mechanical properties of the welds. In this study, different filler metals including duplex, basic, and overalloyed austenitic grades were used with laser-arc hybrid method to weld lean duplex 1.4162 and novel ferritic stainless steels grades 1.4622 and 1.4509. Several sets of joint design and welding parameter combinations were used to adjust the amount of filler metal in the weld. The purpose of the trials was to evaluate whether weld metal microstructures (grain morphology, austenite/ferrite balance, etc.) can be modified by using an applicable joint preparation and an “overmatched” filler metal addition. Weld characterization included several research methods such as: Macro- and microscopic examination using light microscope¸ cross-sectional dilution ratio determination from the metallographic cross sections, electron backscatter diffraction method in order to assess austenite, and ferrite phase proportions in the test welds. The effects of used groove geometry, filler metal composition, and content on resulting metallurgical features of the welds are discussed in detail.

Mechanical and corrosion properties of welded joints in new generation ferritic and duplex stainless steels

This study covers the very preliminary results of welding and characterisation of one novel duplex and two ferritic stainless steel grades. The so-called ”lean duplex” grade EN 1.4162 (UNS S32101) has 21.5% Cr-5% Mn-1.5% Ni-N, the ferritic EN 1.4509 (UNS S43940, AISI 441) 17% Cr, and the other, improved 21% Cr ferritic grade that fulfills also UNS S44330 standard requirements, but has no standard EN designation yet. Both ferritic stainless steels have low (< 0.02%) carbon content and double (Nb+Ti) stabilisation. The materials were used as 1.5 and 2 mm sheets, hence laser and resistance spot welding were selected for welding experiments. The joints were subjected to mechanical testing and critical pitting temperature (CPT) corrosion tests, which were performed on both the base materials and welds. The mechanical tests of the welds did not reveal any significant softening effect due to welding operations. Comprehensive CPT data were achieved for base materials and their welds using two different polarisation potentials, and the new 21% Cr ferritic grade shows a great promise in both as-received and welded conditions. This paper was written as part of the Finnish Metals and Engineering Competence Cluster (FIMECC)’s Demanding Applications (DEMAPP) program.