Prakt. Met. Sonderband 50 (2016) 17 
cannot be completely supressed, i.e. after industrial heat treating operations, there will almost 
always be some amount of sensitisation. 
onary heavy-duty | | . oir . : . 
wufacturer's plant. From this point of view, it cannot be said that the material condition observed in the subj ect parts 1s 
CrNiTi18-10, the out of specification. The martensitic stainless chromium steel X39CrMo17-1, 1.4122, is selected for 
chromium steel springs or similar applications, such as retaining rings, for its combination of moderate corrosion 
lower-chromium resistance and higher strength because it is heat treatable. In some other applications its 
North American polishability is of importance. It is certainly not selected for superior corrosion resistance in 
ve second-source particularly harsh environments; hence it cannot be expected to perform particularly well in such 
;. Also, they were environments. The reported material mix-up therefore only aggravated a pre-existing problem. In 
that the supplier any case, 1.4122 is categorised as superior to 1.4031 and 1.4006 (X12Cr13, DIN 17440. AISI 410) 
hout proper post- in corrosion resistance. 
> subject retaining | . : . . . . . 
and wet cleaning What is more, even the higher-chromium 1.4122 is not corrosion resistant without cathodic 
t. The erroneous protection in applications with notable amounts of chlorides present, such as seawater environments 
or wet cleaning residues. That said, it should be emphasised that the avoidance of the reported 
material mix-up would probably not have prevented the failure from occurring if water remnants 
¢ peak was found from wet cleaning were not completely removed. On the contrary, 1.4122 might be quite sensitive 
thin the corrosion to intergranular attack. In an application like in the case at hand, non-sensitized materials like 
1 this spectrum. It austenitic stainless spring steels are a better material selection. 
ave been a rather | a Ny ; ; 
evented the usual The metallurgical cause of the cracking is intergranular corrosion, or intergranular attack (Kornzer- 
was verified with fall) due to sensitisation of the alloy by heat treatment. Corrosion was apparently induced by wet 
cleaning residues. Contact corrosion between the martensitic snap rings and the austenitic piping 
could have been a contributing factor. 
Eh Also a contributing factor was the flawed material selection, i.e. the material mix-up with the lower- 
dispersive’ Koray chromium steel. Mistakenly X39Cr13, 1.4031, was selected for the retaining rings, instead of the 
mparatively large intended X39CrMo17, 1.4122. The lower chromium content of the former could have aggravated 
erified by STEM- the corrosion problem. In addition, the molybdenum content of the latter would have protected 
sie identification somewhat against pitting, i.e. would have delayed incipient corrosion. It should be emphasized that 
ailure. even 1.4122 is sensitive to this kind of attack; hence, the corrosion-causing agent must be removed 
in any case if the failure is to be prevented from recurring. 
These rather Tara Arguably, the root cause of the failure is the poor choice that had been made in the material 
arbides. This they selection of the retaining rings. From this point of view, the organisational cause of failure is a 
design error. Many design engineers over-estimate the corrosion resistance of a material for a 
particular application, just because the steel is termed "stainless". In the case at hand, the spring 
‚uired for carbide steel X39CrMo17-1, 1.4122, was selected for its combination of mechanical properties and 
nn ca. 450°C and corrosion resistance. However, to achieve the required strength and elasticity, a quench-and-temper 
the final required heat treatment is mandatory. During this heat treatment, particularly upon tempering, secondary 
mperature, if the chromium carbides, mainly of the type Cry3Ce, will precipitate along former austenite grain 
1y, or, finally, by boundaries, sensitising the steel to intergranular attack by depleting of chromium narrow seams 
if the tempering immediately adjacent to the grain boundaries, transforming this part of the matrix into a non- 
strength level, the corrosion resistant material, This happens readily in that particular high-carbon and high-chromium 
ified alloy 1.4122 alloy and is virtually unavoidable. It follows that, given only a moderate corrosive load, the steel 
erain boun daries might fail by intergranular attack, rendering it not so stainless after all.