Nachweis der IK‐Beständigkeit von verschleißfesten Auftragschweißungen mit dem EPR‐Verfahren und mit Salpetersäure‐Flußsäurelösung

Abstract

Tube S 17 8 Si (austenitische Legierung desTyps FeCrl8Ni8Si4Mn mit 2 18% S-Ferrit) gegen interkristalline Korrosion (IK) wurde mit Hilfe der elektrochemischen potentiodynamischen Reaktivierung (EPR), des Salpetersaure-FluBsaure (SF)-Tests nach ASTM A 262 Practice D sowie metallographisch und mikrostrukturanalytisch (REM, EDX) untersucht. Es wurden mit Schutzgas und open arc geschweiote sowie wsrmebehandelte Proben gepruft. Der SF-Test wurde auaerdem in weniger aggressivenvarianten wie 2 M HN03 + 0,5 M HF. 30°C erprobt. In SF besteht kein direkter Zusammenhang zwischen der relativen Korrosionsgeschwindigkeit und IK-Anfalligkeit , was auf eine Inhibition durch Fe-111-Ionen zuruckgefuhrt wird. Durch freies und potentiostatisches Atzen in SF-Losung konnen jedoch chromverarmte Bereiche und zerfallener 6-Ferrit sichtbar gemacht werden. Mit einem optimierten double-loop-EPR-Verfahren (0,5 M H2S04 + 0,01 M KSCN, 30°C) wurde eine Sensibilisierung iiber das Reaktivierungs/Passivierungs-Ladungsverhaltnis in Verbindung mit einer Gefugebeurteilung ermittelt. Die Ergebnisse zeigen, daB die entwickelte EPR-Variante im Gegensatz zu den konventionellen Verfahren zur IK-Prufung bei bis zu 30% S-Ferrit geeignet ist. In geschweifitem Ausgangszustand (SAZ) sind alle Proben bestandig.Temperaturen von 400 "C bzw. 450 "C werden bis mindestens 1000 bzw. 100 h vertragen. Sensibilisierung kann durch Carbid-und o-Phasenausscheidung sowie durch den Zerfall von 6-Ferrit eintreten, dessen Menge und Verteilung dann entscheidend ist. The corrosion resistance against intergranular attack (IGA) of weld claddings from the electrode Tube S 17 8 Si (alloy type FeCrl8Ni8Si4Mn with 2 18% &ferrite) has been investigated using electrochemical potentiodynamic reactivation (EPR) technique, the nitric-hydrofluoric acid (SF) test according to ASTM A 262 practice D, metallographic and microstructure analytical methods (SEM, EDX). Samples produced by different welding techniques (shielded arc and open arc) and partly additional thermal treated have been investigated.In standardized SF-solution and in 2 M HN03 + 0.05 to 0.5 M HF at 30°C there is no correspondence between corrosion rate and IGA-susceptibility, possibly caused by inhibitive action of Fe-111-ions. SF etchants evaluate Cr-depleted zones and decomposed S-ferrite. In the optimized EPR double loop test in 0.5 M H2S04 + 0.01 M KSCN at 30°C the extend of sensitization is indicated by the ratio of reactivation to passivation charge with additional assessment of the microstructure. It is shown that the EPR-test in the evaluated manner is able to find out IGA at claddings up to 30% &ferrite. In the as-welded state all claddings are resistant. Treatments at 400°C resp. 450°C were tolerated up to at least 1000 resp. 100 h. Sensitization may be caused by precipitation of Cr-rich carbides and by decomposition of &ferrite, for IGA the amount and distribution of ferrite plays a decisive role.

Functional Surface Layers on Stainless Steels Generated by Means of EB Cladding and Nitrocarburizing

“…A different technology for the specific surface modification of stainless steels with respect to the improvement of surface hardness and wear resistance is the surface cladding by means of built‐up welding processes, e.g., shielded metal arc (SMA) welding or metal inert gas (MIG) welding, plasma transferred arc (PTA) welding as well as laser cladding or electron beam cladding . A range of additional materials that exhibit specific properties, especially with respect to highly abrasively stressed areas of components, can be locally cladded.…”

“…A range of additional materials that exhibit specific properties, especially with respect to highly abrasively stressed areas of components, can be locally cladded. The main surface cladding methods reported in the literature are those for applying additive materials based on Fe or Co to corrosion‐resistant steels . The application of Fe‐based additives leads to surface claddings whose thicknesses are in the range of 0.6 to 2 mm, with high hardness and improved wear resistance compared to the untreated base material .…”

“…The application of Fe‐based additives leads to surface claddings whose thicknesses are in the range of 0.6 to 2 mm, with high hardness and improved wear resistance compared to the untreated base material . At the same time the formation of carbides of the type M 7 C 3 , M 3 C, M 5 C 2 results in hardness values between 420 and 1100 HV . The increase in hardness values within the surface claddings correlates with a significant improvement in abrasive wear behavior .…”

“…However, it has to be considered that Cr is bonded, due to a possible carbide formation within these layers, and is therefore not effectively contributing to passivation. So in the case of inappropriate additive material selection, e.g., with a Cr content that is too low, the corrosion behavior is strongly impaired when compared to the corrosion‐resistant base material . Co‐based additives exhibit a very good or even better corrosion resistance than stainless steels .…”

“…Depending on the technological parameters and the resulting alloying content, the hardness values vary between 400 HV0.5 and 1400 HV0.5 . Comparing both additive materials with respect to the resulting corrosion resistance of the surface claddings on corrosion‐resistant steels it can, in general, be observed that Co‐based additives exhibit much better corrosion behavior, especially in acidic environments or media …”