What is grade 316Ti (1.4571)?
Grade 316Ti stainless steel has been traditionally specified by German engineers and users with the Werkstoff number 1.4571.The former steel grade in the UK was 320S31.
This grade is essentially a standard carbon 316 type with titanium stabilisation and is similar in principle to the titanium stabilisation of the 304 (1.4301) type to produce 321 (1.4541). The addition of titanium is made to reduce the risk of intergranular corrosion (IC) following heating in the temperature range 425-815C.
Intergranular corrosion
When austenitic stainless steels are subject to prolonged heating in the temperature range 425-815C, the carbon in the steel diffuses to the grain boundaries and precipitates chromium carbide. This removes chromium from the solid solution and leaves a lower chromium content adjacent to the grain boundaries. Steels in this condition are termed 'sensitised'. The grain boundaries become prone to preferential atack on subsequent exposure to a corrosive environment. This type of corrosion is known as intergranular corrosion (IC), also known in the past as 'weld decay'.
The addition of titanium reduces the risk of IC since titanium carbo-nitrides are formed in preference to chromium carbides which has the effect of maintaining the correct distribution of chromium throughout the structure of the steel.
The result is that areas adjacent to grain boundaries, where the carbo-nitrides form, is not depleted of chromium to a level at which localised corrosion can occur in the grain boundary area.
The alternative approach to reducing the risk of IC attack is to reduce the carbon level to below 0.03%. In this way a grade of 316 is produced with, in practice, the same resistance to IC as 316Ti (320S31/1.4571) . This is the basis of the 316L types (316S11/1.4404 and 316S13/1.4432).
Is 316Ti interchangeable with 316L?
Under most conditions it can be taken that the two grades are interchangeable, 316L (316S11/1.4404) being suitable for applications where 316Ti (320S31/1.4571) is specified. In aqueous corrosion media or environments at ambient temperatures, there is no practical advantage in specifying the 316Ti type in preference to the 316L. In some circumstances the 316L (1.4404 / 1.4432) grades may be better choices.
Mechanical Properties
The presence of titanium to 1.4571 does, however, give some improvements to mechanical strength, especially, at elevated temperatures above about 600 C. and care must therefore be exercised in selecting 1.4404 as a substitute under these conditions. The 1.4571 may however have inferior impact properties at ambient temperatures, compared to the 1.4404 / 1.4432 types.
Machinability
The machinability of 1.4571 can also be an issue as the titanium carbo-nitrides particles can result in higher tool wear and may not cold form or cold head as readily as the 1.4404 / 1.4432 types.
Polishing
The titanium carbo-nitrides in 1.4571 can also result in problems where high standards of polished surface finish are required. The titanium carbo-nitrides particles can result in 'comet-tail' streaks on the polished surface as they are dragged out during polishing. This is similar to the 1.4541(321) grade, which was not recommended for a 'No8' bright mechanical polish in now obsolete BS1449 Pt2 (now replaced by BSEN 10088:2-1995 finish 1P/2P).
Corrosion Resistance
There is also some evidence that the 1.4571 type may have inferior pitting and stress corrosion cracking resistance, compared to the 1.4404 / 1.4432 types, although the general corrosion resistance can be assumed to be generally similar. The titanium stabilised 1.4571 grade may also be prone to 'knife line attack' in the heat-affected zones of welds, very close to the fusion zone where the carbo-nitrides have redissolved in the solid steel matrix.
Weldability
The weldability of the 1.4571 and 1.4404 / 1.4432 can be assumed to be similar. Neither of the grades can be expected to weld 'easier' or 'better' than-the-other. Niobium stabilised fillers (welding consumables) should be used for welding the 1.4571, especially where elevated temperature weld strength may be important. In other circumstances a '316L' filler should give a matching weld metal aqueous corrosion resistance to that of the 'parent' 1.4571 '316Ti' material.
Further Information
There is also a relevant article on the comparison between these grades on the Euro Inox website.
Source: http://www.bssa.org.uk/
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