Selective Laser Melting of Duplex Stainless Steel 2205: Effect of Post-Processing Heat Treatment on Microstructure, Mechanical Properties, and Corrosion Resistance

Additive manufacturing (AM) is a rapidly growing field of technology. In order to increase the variety of metal alloys applicable for AM, selective laser melting (SLM) of duplex stainless steel 2205 powder and the resulting microstructure, density, mechanical properties, and corrosion resistance were investigated. An optimal set of processing parameters for producing high density (>99.9%) material was established. Various post-processing heat treatments were applied on the as-built predominantly ferritic material to achieve the desired dual-phase microstructure. Effects of annealing at temperatures of 950 °C, 1000 °C, 1050 °C, and 1100 °C on microstructure, crystallographic texture, and phase balance were examined. As a result of annealing, 40–46 vol.% of austenite phase was formed. Annealing decreased the high yield and tensile strength values of the as-built material, but significantly increased the ductility. Annealing also decreased the residual stresses in the material. Mechanical properties of the SLM-processed and heat-treated materials outperformed those of conventionally produced alloy counterparts. Using a scanning strategy with 66° rotation between layers decreased the strength of the crystallographic texture. Electrochemical cyclic potentiodynamic polarization testing in 0.6 M NaCl solution at room temperature showed that the heat treatment improved the pitting corrosion resistance of the as-built SLM-processed material

Viivästynyt murtuminen metastabiileissa matalanikkelisissä austeniittisissa ruostumattomissa teräksissä

Metastable austenitic stainless steels can be susceptible to delayed cracking after forming processes. The objective of this research was to explain the high susceptibility of low-Ni high-Mn austenitic stainless steels to delayed cracking, and to clarify the role of the different contributing factors, i.e. solute hydrogen, strain-induced α'-martensite and tensile residual stresses, in the phenomenon. Susceptibility of seven austenitic stainless steels, both conventional Fe-Cr-Ni grades and low-Ni grades, to delayed cracking was investigated by means of deep drawing Swift cup tests. Residual stresses in the cups were measured with X-ray diffraction and a ring slitting method. Volume fraction of strain-induced α'-martensite was determined with a Ferritescope. Hydrogen content of the test materials was analysed with hot extraction, melt extraction and thermal desorption spectroscopy. Hardness of α'-martensite and austenite phases was measured using nanoindentation. Additionally, a constant load tensile testing arrangement was developed and applied for systematic study on the role of different contributing factors in delayed cracking kinetics. The presence of α'-martensite was a necessary prerequisite for delayed cracking to occur in austenitic stainless steels with typical internal hydrogen concentrations (< 5.5 wppm). Cracking proceeded through α'-martensite phase. Martensitic transformation substantially increased the magnitude of residual stresses in deep-drawn cups. Alloying elements of the stainless steels influenced the sensitivity to delayed cracking through their effect on the austenite stability and properties of α'-martensite. According to nanoindentation measurements the hardness of α'-martensite correlated with the level of residual stresses. Critical combinations of residual stress, α´-martensite and hydrogen content for delayed cracking were specified for each test material. Explanations for the high susceptibility of low-Ni grades were their high residual stresses after forming, and high internal hydrogen content. Lowering the hydrogen content by annealing markedly lowered the risk of delayed fracture. Constant load tensile testing results demonstrated the role of α'-martensite as a medium for hydrogen diffusion. Hydrogen content seemed to have the strongest effect on time to fracture, which supports the assumption that cracking requires hydrogen accumulation at regions of high stress. The results of this research work can be utilized in the design of demanding forming applications, to avoid the risk of delayed fracture. Increased understanding of the delayed cracking phenomenon enables the development of novel cost-efficient, high-strength stainless steels and wider usage of current austenitic stainless steels.Metastabiilit austeniittiset ruostumattomat teräkset voivat olla alttiita viivästyneelle murtumalle muovauksen jälkeen. Tämän tutkimuksen tavoitteena oli selittää matalan nikkelipitoisuuden austeniittisten ruostumattomien terästen korkeaa alttiutta viivästyneelle murtumiselle ja selvittää ilmiöön vaikuttavien tekijöiden, kuten materiaaliin liuenneen vedyn, muovauksen aiheuttaman α'-martensiitin ja jäännösjännitysten merkitys. Seitsemän austeniittisen ruostumattoman teräksen, sekä Fe-Cr-Ni- että matalanikkelisten lajien, alttiutta viivästyneelle murtumiselle tutkittiin Swiftin kuppikokeiden avulla. Syvävedetyistä kupeista määritettiin jäännösjännitykset röntgendiffraktiolla ja jännitysten laukeamiseen perustuvalla menetelmällä. Muovauksen aiheuttaman α'-martensiitin määrä mitattiin ferriittimittarilla. Koemateriaalien vetypitoisuus määritettiin kolmella eri menetelmällä. Muovatuista materiaaleista mitattiin α'-martensiitin ja austeniitin kovuus nanoindentaatiolla. Lisäksi kehitettiin koejärjestely vakiovoima-vetokeisiin, joilla tutkittiin systemaattisesti eri tekijöiden vaikutusta viivästyneen murtumisen kinetiikkaan. Viivästynyttä murtumista ei tapahtunut stabiileissa austeniittisissa ruostumattomissa teräksissä, joihin ei muodostunut muovauksessa α'-martensiittia, tyypillisillä valmistuksen jälkeisillä vetypitoisuuksilla (< 5.5 wppm). Metastabiileissa materiaaleissa viivästyneet murtumat etenivät α'-martensiittifaasia pitkin. Nanoindentaatiomittausten perusteella α'-martensiitin kovuus korreloi jäännösjännitystason kanssa. Tutkittaville materiaaleille määritettiin kriittiset jäännösjännitysten, α'-martensiitin ja vetypitoisuuden tasot, jotka aiheuttavat viivästyneitä murtumia. Matalan nikkelipitoisuuden terästen korkeaa alttiutta viivästyneelle murtumiselle selittävät niiden korkea jäännösjännitystaso ja korkea vetypitoisuus. Vetypitoisuuden alentaminen hehkutuksella alensi merkittävästi murtumisriskiä. Vakiovoima-vetokokeiden tulokset havainnollistivat α'-martensiitin roolia vedyn diffuusiossa. Vetypitoisuudella oli suuri vaikutus murtumisaikaan, mikä tukee käsitystä, että murtuminen edellyttää vedyn kerääntymistä jännityskeskittymien alueelle. Tämän tutkimuksen tuloksia voidaan hyödyntää viivästyneen murtumisen välttämiseksi suunniteltaessa vaativia muovaussovelluksia. Ilmiön syvällinen ymmärtäminen tukee uusien kustannustehokkaiden, lujien ruostumattomien terästen kehittämistä sekä olemassa olevien teräslaatujen laajempaa käyttöä

Delayed cracking of metastable austenitic stainless steels after deep drawing

VK: T20309Peer reviewe

Inspection of Carbon Fibre – Titanium – Carbon Fibre Stepped-Lap Joint

The optimal combined use of composites and metallic material is essential for the performance of modern tactical aircrafts. At the same time, the resulting structures include complex metal-to-composite joints that may develop failures during use. The inspection of such structures is often highly demanding. The present paper discusses the complex inspection of stepped-lap joint between carbon fibre wing panel and titanium attachment of tactical aircraft. The joint has been found to be susceptible to disbonding both in the outer and inner titanium to carbon fibre interfaces. Due to the multi-material multi-layer structure, the inspection is particularly demanding, especially for the inner interface. Successful inspection can be developed by careful analysis of the ultrasonic signal. A flawless joint on the outer interface will result in phase change at the interface, that can be seen on the RF signal. A disbond prohibits this phase change and thus can be detected by noting the absense of this phase change. Defects bigger than 4 mm can be detected using this effect. Disbond on the outer surface will prohibit sound from traveling to the inner surface. Consequently, disbond on outer surface can be detected by noting a decreased amplitude in the inner-interface echo. A disbond on the inner surface will, likewise, prohibit sound from going through the interface and thus increase reflection from that interface. Consequently, a disbond on the inner surface can be detected by noting an increase in the amplitude of the inner-interface echo. Thus disbonds on both inner and outer surfaces can be detected by monitoring possible decrease or increase in the inner surface echo indicating disbond in the outer or inner surface, respectively.Peer reviewe