Effect of Mg/Si Ratio on Hardening Behavior, Microstructure and Properties of Al-Mg-Si Alloys

The effects of Mg/Si ratio on precipitation behaviour and properties of Al-Mg-Si alloys were studied by using electrochemical test, corrosion test and transmission electron microscope (TEM). The results show that with the increases of Mg/Si ratio from 0.9 to 1.1, the density of the β" decreases, and the mechanical properties decrease. When the ratio of Mg/Si increases from 1.0 to 1.1, the density y of β" does not increase significantly, but the continuous degree of the MgSi phase decreases significantly. The source of cracks originate from MgSi phase, which reduces the mechanical properties. When the Mg/Si ratio is 0.9, the alloy is in an over-Si state, which results in serious intergranular corrosion (IGC).

Transmission Electron Microscopy Study on the Precipitation Behaviors of Laser-Welded Ferritic Stainless Steels and Their Implications on Intergranular Corrosion Resistance

The intergranular corrosion susceptibility of ferritic stainless-steel weldments is strongly dependent on chromium carbide precipitation phenomena. Hence, stabilization is widely used to mitigate the aforementioned precipitation. In contrast, stabilization has proved ineffective to fully prevent intergranular corrosion due to segregation of unreacted chromium during solid-state heat-treatments. To analyze the precipitation behavior of 17 wt.-% chromium ferritic stainless steels during laser welding, sheets of unstabilized and titanium-stabilized ferritic stainless steels were welded in a butt joint configuration and characterized with special consideration of precipitation behavior by means of transmission electron microscopy. While unstabilized ferritic stainless steels exhibit pronounced chromium precipitate formation at grain boundaries, titanium-stabilization leads to titanium precipitates without adjacent chromium segregation. However, corrosion tests reveal three distinctive corrosion mechanisms within the investigated ferritic stainless steels based on their inherent precipitation behaviors. In light of the precipitation formation, it is evident that immersion in sulfuric acid media leads to the dissolution of either grain boundaries or the grain boundary vicinity. As a result, the residual mechanical strength of the joint is substantially degraded.

Investigation on Impact of Heat Input on Microstructural, Mechanical, and Intergranular Corrosion Properties of Gas Tungsten Arc-Welded Ti-Stabilized 439 Ferritic Stainless Steel

In the present study, gas tungsten arc welding was employed to weld Ti-stabilized 439 ferritic stainless steel using 308L austenitic stainless steel filler electrode with varying heat input, i.e., low heat input (LHI) and high heat input (HHI). The optical microstructure revealed the formation of retained austenite (RA) and ferrite in the weld zone (WZ), whereas the peppery structure consisting of chromium-rich carbides were observed in the heat-affected zone for both the weldments. The volumetric fraction of RA was calculated using X-ray diffraction analysis. The RA’s content decreased, whereas grain size in WZ increased with an increase in heat input. The local misorientation and grain boundary distribution in the welded region was investigated by electron backscattered diffraction. The LHI weldment depicted the higher micro-hardness and tensile strength attributed to the higher content of RA as compared to HHI; however, the opposite trend was observed for the intergranular corrosion resistance.

The Importance of Local Chemistry and Potential in Localized Corrosion and Stress-Corrosion Cracking

The nature and rates of the chemical and electrochemical reactions that occur within the occluded regions of a given alloy are controlled by the local electrochemical potential and the local solution composition. The very small physical dimensions of these regions lead to challenges in both measurement and modeling. When performed in a coordinated and complementary way, measurements and modeling provide insights into the controlling processes of a range of localized corrosion phenomena, including crevice corrosion, pitting, intergranular corrosion, and stress-corrosion cracking. Examples of attempts to overcome the measurement challenges are described for a range of corrosion scenarios, including identification of the critical ionic species in stainless steel crevice corrosion and in the corrosion of aircraft lap joints, operando measurement of chemistry and potential simultaneously within stress-corrosion cracks, and monitoring of water layer thickness in salt spray testing. Examples of work addressing the challenges in modeling localized corrosion including intergranular corrosion of AA5XXX alloys, scaling laws in crevice corrosion, the extent to which the Laplace Equation can be used and applied to geometrically complex galvanic structures, and an approach to modeling localized corrosion for extraordinarily long service times. Finally, suggestions regarding future avenues of research are provided.

A Novel Approach to Inhibit Intergranular Corrosion in Ferritic Stainless Steel Welds Using High-Speed Laser Cladding

Ferritic stainless steels are prone to localized corrosion phenomena such as pitting corrosion or intergranular corrosion, in particular when jointed by fusion welding processes. State-of-the-art techniques to avoid intergranular corrosion mainly consist of alternating alloy concepts or post-weld heat-treatments—all of which are associated with increased production costs. Hence, the present investigation seeks to introduce a novel approach for the inhibition of intergranular corrosion in ferritic stainless steel welds through the use of high-speed laser cladding. Here, vulnerable sites prone to intergranular corrosion along the weld seam area are coated with a chemically resistant alloy, whereby an overlap is achieved. Optical and electron microscopy as well as computer tomography and tensile tests reveal that the detrimental effects of intergranular corrosion in both stabilized and unstabilized ferritic stainless steel are substantially reduced. In addition to that, the effects of varying overlap widths on the identified corrosion phenomena are studied. Moreover, the resulting dilution and precipiation phenomena at the clad–sheet interface are thoroughly characterized by electron backscatter diffraction and energy dispersive X-ray spectroscopy, whereby interrelationships to corrosion resistance can be drawn. As a result of this investigation, the number of techniques for the inhibition of intergranular corrosion is enlarged, and substantial cost-saving potentials in the manufacturing industry are unlocked.

OUTOKUMPU MODA 409/4512

Outokumpu Moda 409/4512 is a weldable, titanium-stabilized 11.5% chromium ferritic stainless steel with good oxidation resistance in dry air. Because of its titanium alloying, Outokumpu Moda 409/4512 can be welded in all dimensions without becoming susceptible to intergranular corrosion. It is possible to use Outokumpu Moda 409/4512 at elevated temperatures, for example in automotive exhaust systems, where it is often used to replace aluminum-coated carbon steel. This datasheet provides information on composition, physical properties, elasticity, and tensile properties. It also includes information on corrosion resistance as well as casting, heat treating, machining, and joining. Filing Code: SS-1342. Producer or source: Outokumpu Oyj.

Effect of Sensitization Time on Intergranular Corrosion of 347H Stainless Steel

In this paper, 347H stainless steel of three situation of solution, primary and secondary stabilizing with 0, 12, 24, 48 and 72 h accelerated sensitization heat treatment. The change of sensitization degree with time was studied by metallographic test, double-loop electrochemical potentiodynamic reactivation (DL-EPR) test, intergranular corrosion test and microscopic morphology observation. The result shows that the sensitization degree of the solution material increases rapidly and reaches the maximum value after 12h sensitization heat treatment. After that, it still belonged to severe sensitization situation, but the index gradually decreased. After stabilizing heat treatment, the sensitization degree of the material is lower than the situation of solution. After heat treatment for 48h, the material located on “possible sensitization” range, and the sensitization degree of the secondary stabilized material was always lower than is of the primary. It indicates that the sensitization of materials can not be completely inhibited by stabilizing heat treatment, and other anti-corrosion measures should be considered.