Welding "Grade 91" Alloy Steel

This information is provided by Sperko Engineering based on information currently possessed by Sperko Engineering; Sperko Engineering

accepts no responsibility for proper application of this information or for any consequential damages associated with application of this

information.

© Sperko Engineering Services, Inc. September 2002, Page 1 of 1

One of the materials that has spread through the piping and boiler industry recently is an alloy,

referred to in various specifications as “T-91, “P-91, “F-91” and “Grade 91.” This is a specially

modified and heat treated 9% chromium, 1% Molybdenum, Vanadium enhanced (9Cr-1MoV) steel

that performs quite well at elevated temperature – usually 1000°F and higher. It was first used in

the mid-1980s and has “picked up steam” since then. If you are going to weld or fabricate Grade 91

alloys, beware! These are not your father’s chrome-moly steels!

Development of Grade 91 began in 1978 by Oak Ridge National Labs for the breeder reactor and

further developed by other researchers since then. Other grades such as grade 92 and grade 911

(who says metallurgists don’t have a sense of humor?) are also under development.

Since P/T-91 is modified with vanadium, nickel, aluminum, niobium and nitrogen, it develops very

high hardness. Tramp residual elements in this steel, such as phosphorous, sulfur, lead, tin, copper,

antimony and other elements will segregate to the grain boundaries during solidification of the weld,

and, since the weld metal is very hard, it will crack quite easily. It is, therefore, very important to

use low residual filler metal.

For SMAW, E9015-B9 electrodes are preferred. EXX15 type electrodes have no extra iron powder in

the coating like EXX18 electrodes, eliminating one source of contaminants. Stay away from E9018-

B9 electrodes when welding Grade 91 steels unless the supplier can guarantee that the weld metal

has low tramp residuals. If you routinely get crater cracks (also known as “solidification anomalies”

and “rogue weld metal’), the filler is not low in residuals and you should send it back (or at least get

some good stuff). Look carefully for crater cracks, and keep in mind that one batch of electrodes from

a manufacturer may crack and another batch not crack. Two trade names of electrodes and filler

that have low residuals are Metrode Chromet 9B9 electrode and Euroweld 9CrMoV wire. The wire

is suitable for GTAW, GMAW and SAW (with a suitable flux, such as Lincoln MIL800H, Lincoln 882,

Thyssen Marathon 543, Bavaria-Schweisstechnik WP380 and Oerlikon OP 76). Welding Grade 91

using FCAW requires even more care since many FCAW wires do not provide adequate toughness at

70°F (the lowest hydrostatic test temperature permitted by ASME); the only FCAW wire that

consistently provides more than 20 Ft-lbs absorbed energy at 70°F is Metrode’s Supercore F91. The

above electrodes and filler metals are available from stock at Euroweld at 1-704-662-3993 or

www.euroweld.com.

The performance of Grade 91 welds depends entirely on having the correct chemical analysis in the

weld metal; therefore, it is highly recommended that filler metals be purchased with test reports

showing actual chemical analysis for the specific heat/lot combination that one has purchased. In

addition, a minimum carbon content of 0.09%, a minimum niobium content of 0.03%, (although

slightly lower Niobium can be accepted with flux cored wire since titanium is an effective substitute

for Niobium) and a minimum nitrogen of 0.02% should be specified to ensure adequate creep

strength in the weld metal.. In addition, the sum of Mn + Ni should not exceed 1.5%. Manganese

and nickel depress the lower transformation temperature, and as it exceeds 1.5%, the transformation

temperature drops below 1450ºF, narrowing the range in which heat treatment can be done safely.

In addition, the Mf temperature goes down, increasing the possibility of retained austenite after

PWHT.

When using SAW, a basic flux is preferred since other flux types will burn out carbon and permit

elevated oxygen and nitrogen levels reducing the strength and toughness of the weld metal.

Since this is a highly-hardenable alloy, it is subject to hydrogen cracking. Purchase of E9015-B9-H4

electrode is recommended. The “H4” designation indicates that the electrode exhibits less than 4 ml

of hydrogen per 100 grams of weld metal. This is truly a very low hydrogen electrode – exactly what

is best for welding highly-hardenable steel like Grade 91. Even with diffusible hydrogen control of

the electrodes, it is recommended that the electrodes be stored in heated portable rod boxes at the

welding location rather than just distributed in the normal fashion. SAW wire/flux combinations

and FCAW wire should be ordered with “-H4” designations also, although FCAW wire may not be

available except as H-8.

Preheat and interpass temperature are very important. A range of 400 to 550°F is recommended,

After welding is completed, the joint should be allowed to cool slowly to at least 200°F after welding

is completed to be sure that all the austenite has been transformed to martensite prior to postweld

heat treatment (PWHT). If this is not done, there is risk of martensite formation after PWHT; this

will result in hard, brittle welds. For the metallurgists out there, the Mf temperature is above 212°F,

varying some with the grain size.

The welding technique is also important. Since a wide, flat bead is best, a slight weave technique

and high travel speed should be specified. Ropy beads are bad since tall, narrow beads tend to crack.

Concave beads should also be avoided, particularly with SAW. Bead thickness should not exceed 1/8

in. for SMAW and FCAW to promote tempering of previous passes. These conditions of welding

should specified in the WPS to provide correct guidance to welders, not to give them a hard time. Be

sure that your welders have been trained regarding these special requirements and that they comply

with them.

Finally, postweld heat treatment is required for Grade 91 steels, regardless of what construction

codes may permit. The holding range should be 1375 to 1425°F for a minimum of 2 hours. Even on

small superheater tubes, a long time at temperature PWHT temperature is necessary to form the

required weld structure, to ensure adequate toughness during hydrostatic testing and to ensure

adequate service life. The lower transformation temperature can be as low as 1450°F; if this

temperature is exceeded during PWHT, the weld should be allowed to cool to below 200°F followed

by reheat treating or the condition of the joints should be evaluated by hardness testing. Refer to

AWS D10.10, Recommended Practice for Local Heating of Welds in Piping and Tubing, for excellent

direction on locating and attachment of thermocouples, the extent of insulation needed, heating coils

arrangement, etc. if local heating (preheat, postweld baking, PWHT, etc) is going to be done.

After PWHT, the weld hardness should be in the range of 200 to 275. Hardness up to 300 Brinnell

may be accepted, but any hardness over 300 is an indication of inadequate PWHT. SMAW and SAW

weld metal will exhibit higher hardness when compared to GTAW and FCAW. Hardness below 175

indicates overheating of the joint, and such joints should either be replaced or the part should be

normalized and tempered.

Do not perform hardness tests that will leave deep impressions in the surface of thin tubes. When

performing hardness tests, it is important to prepare the surface properly, particularly for HAZ

readings. Since the base metal may have a layer of decaraburization on the surface, about 1/32 inch

of metal should be removed by grinding, and that should be followed by polishing to a 120 grit finish.

This preparation will also make readings more consistent and should also be followed when

measuring the hardness of the weld metal.

Grade 91 can be hot bent using furnace heating or induction heating between 1600 and 2000°F, but

the low end of this range is preferred. Pipes that are hot bent should be given a full-furnace

normalizing heat treatment at 1900 to 1950°F for 30 minutes per inch of wall thickness, air cooled to

below 200°F and tempered in the PWHT range of 1375 to 1425°F for 1 hour per inch of thickness.

Cold bent pipe should be given a stress-relieving heat treatment at the above tempering temperature

for 15 minutes per inch of wall thickness.

Another strange phenomenon with Grade 91 is that it is subject to stress-corrosion cracking in the

as-welded condition. The media has not been identified as yet, and it does not happen for several

days after the weld has cooled to ambient, but it does happen. Of specific concern is shop-fabricated

This information is provided by Sperko Engineering based on information currently possessed by Sperko Engineering; Sperko Engineering

accepts no responsibility for proper application of this information or for any consequential damages associated with application of this

information.

© Sperko Engineering Services, Inc. September 2002, Page 2 of 2