MP35N / UNS R30035 (also ATI 35N): Meeting NACE MR0175 / ISO 15156 Requirements

This is the third in a series of articles on this unique material – Article 1 introduced UNS R30035 and gave an overview of the material, while article 2 considered AMS 5844 / AMS 5845 material in more detail.

This article will consider material produced to meet the requirements of NACE MR0175 / ISO 15156.

UNS R30035 is the highest strength alloy approved in NACE MR0175 / ISO 15156 for use in sour service conditions.

The chemical make-up of UNS R30035 and, in particular, the combination of the major corrosion resisting elements of Cobalt (around 33% content), Nickel (33-37%), Chrome (19-21%) and Molybdenum (9-10.5%) make the material stand out where resistance to pitting, crevice and stress corrosion cracking in harsh environments (particularly hydrogen sulphide, salt water and other chloride environments) or attack from mineral acids (nitric, hydrochloric and sulphur) is expected.

NACE MR0175 / ISO 15156 permits the use of UNS R30035 processed per the “NACE” specification in any combinations of temperature, under H2S partial pressure or where chloride concentration or pH occurs in downhole production environments. To comply with NACE MR0175 / ISO 15156, the material must be cold drawn and final aged, with a hardness of less than 51 HRC.

All the “NACE” type material of UNS R30035 is compliant with NACE MR0175 / ISO 15156 and is 34-51 HRC. This does mean that machinists and manufacturers will have to machine their product out of aged bar (unlike cold drawn AMS 5844 which can be aged up to AMS 5845 after machining).

One further issue we have come across is the number of cold work reduction and final ageing times and temperatures that provide a wide range of material strengths – all of which meet the NACE MR0175 / ISO 156 requirements – and this does produce a number of “options” for our customers to consider.

Properties from ageing times / temperatures available for bar stock that meet the requirements of NACE MR0175 / ISO 15156:

	Guaranteed minimum
Size	UTS (ksi)	YS (0.2%) ksi	Elong % (4D)	RA %	HRC
Age at 1300 deg F for 4 hours
<2” diameter	220	210	10	40	51 max
>2” diameter	185	175	12	40	51 max
Age at 1350 deg F for 4 hours
<2” diameter	210	200	10	40	51 max
>2” diameter	175	165	12	40	51 max
Age at 1425 deg F for 6 hours
<2” diameter	190	180	10	40	51 max
>2” diameter	170	160	12	40	51 max

As you will see, for material developed for the oil & gas industry and complying with the requirements of NACE MR0175 / ISO 15156 there is a trade-off in strength compared to the AMS grades (which can go up to 230 ksi / 1585 Mpa) in order to maximise the excellent stress corrosion cracking resistance for harsh environments.

We show in the table the range of options available, although Broder Metals Group have found over the 4+ years that we have stocked UNS R30035 that the majority of  our customers prefer to have a toughness and strength in the material (Yield Strength above 200 ksi, for example) alongside the superb corrosion resistance (therefore prefer an ageing of 1300 degree F for 4 hours).  We aim to stock material that meets these requirements, although there are a few end user specifications (particularly for fasteners) that require a specific ageing time and temperature (most notably 1425 degrees F for 6 hours) and we will try to hold stock to meet these specifications in cooperation with our customers wherever we can.

The other limiting factor for manufacturers to consider is that bar material currently only goes up to a maximum of 3” (76.2 mm) diameter. However, UNS R30035 remains one of the strongest and toughest materials for demanding corrosive environments.

Articles 4 and 5 will look at how to machine and weld UNS R30035.

Broder Metals Group hope that these articles are useful and would welcome enquiries for our stock range and any questions not covered.

Copyright © 2019 Broder Metals Group
Acknowledgement: all values quoted are courtesy of Carpenter Technology Corporation: “MP35N: A Superalloy for Critical OIL and Gas Applications”