Alloy 625 Grade 1 vs Grade 2: a heat-treatment condition, not a quality level

In many material specifications a higher grade number means more strength or better quality. Alloy 625 (UNS N06625) does not work that way. Order Grade 2 on the assumption that it is one step up from Grade 1, and you receive material with a yield strength one third lower.

The grade is the final heat treatment

ASTM splits Alloy 625 into two grades, and the only difference is the temperature of the last heat treatment before delivery.

The strengths shown are for seamless pipe and tube (ASTM B444). Other product forms follow in the next section.

A higher solution temperature grows a coarser grain. Coarse grain gives up some strength at room temperature but resists creep better under long-term load at high temperature. The two grades therefore serve two kinds of duty:

• Corrosion-resistant structures from ambient to 1100 °F, for example seawater, chemical process and downhole service: choose Grade 1 for its strength.
• Parts carrying load for long periods above 1100 °F, for example furnace hardware and exhaust systems: choose Grade 2.

After solution annealing, Grade 2 may also be given a stabilisation anneal at 1800 °F (982 °C) minimum to improve resistance to sensitisation. The specifications allow it with or without this step, so state it on the order if you need it.

The minimum values depend on the product form

Material marked Grade 1 does not carry the same minimum strength as sheet, as heavy plate and as large bar. The table below was checked against each product specification. Every cell gives tensile strength in ksi, yield strength in ksi and elongation in %, all minimum values.

In SI units as printed in the specifications: 120 ksi is 827 MPa, 110 is 758, 100 is 690, 60 is 414, 55 is 379, 50 is 345 and 40 is 276. B705 alone prints 825, 415 and 275 for the 120, 60 and 40 ksi values.

Three things to notice:

• Grade 1 has a lower tier. Hot-rolled plate, large bar and heavy-section forgings carry a 110 ksi tensile minimum, not 120.
• Grade 2 is the same in every product form: 100 ksi tensile and 40 ksi yield.
• Bar has a Grade 3. Solution annealed and then cold worked, up to 2.5 in. (63 mm), with minimums of 135 ksi (930 MPa) tensile, 100 ksi (690 MPa) yield and 25% elongation. It exists only in the bar specification B446. Plate, pipe and forgings have no such grade.

Aerospace AMS specifications set their own requirements. The full comparison by product form is on the Alloy 625 grade page.

No grade on the order means Grade 1

The ordering clauses of all seven specifications carry the same sentence: if no grade is specified, Grade 1 will be supplied.

So if a high-temperature duty needs Grade 2 and the order says only “N06625, ASTM B444”, a supplier who ships Grade 1 has met the order in full. The reverse also matters. Write Grade 2 for an ambient corrosion duty and the material arrives with the lower strength minimum, so design values taken from Grade 1 no longer hold.

Five details that are easy to miss

The SI figures are not the standard values. The plate, tube, bar, forging and welded tube and pipe specifications each state that inch-pound units are the standard and the SI values in parentheses are conversions for information only. The same 120 ksi is printed as 827 MPa in B704 and as 825 MPa in B705. When acceptance is argued on a metric test report and the result sits in that gap, the inch-pound value governs.

A fitting takes its grade from its own heat treatment. Under ASTM B366, Grade 1 fittings and Grade 2 fittings may both be made from starting material of either grade. The difference lies in testing. Grade 1 fittings must be tension tested per lot. Grade 2 fittings are exempt only when the raw-material test report designates Grade 2 and the final heat treatment of the fitting meets the solution-annealing requirement. The grade on the raw-material certificate is not the grade of the fitting.

The hydrostatic test stress differs. For B444 seamless pipe and tube, the test pressure is calculated from an allowable fibre stress of 30 000 psi (207 MPa) for Grade 1 and 25 000 psi (172 MPa) for Grade 2.

Some material carries no mechanical guarantee. “Hot forming quality” pipe and tube and “forging quality” bar are supplied to chemical composition and surface inspection only, with no tensile test. Very thin sheet and strip is also exempt: yield strength does not apply below 0.020 in. (0.508 mm) and elongation does not apply below 0.010 in. (0.254 mm).

Reheating changes the condition. The specifications carry a note that hot working or reannealing may change properties significantly, depending on working history and temperatures. If you buy Grade 2 and hot form it yourself, the condition after forming has to be re-established.

In titanium, a grade is a different alloy

The ASTM titanium specifications also use the word, but there each grade is a separate alloy with its own UNS number and chemistry. These are the most common ones in the plate specification ASTM B265. Strengths are in ksi with MPa in parentheses.

• For unalloyed Grades 1 to 4, a higher number is stronger, because more oxygen and iron are allowed. The price is elongation falling from 24% to 15% and harder forming. A higher number is still not a higher quality level.
• Grade 23 is weaker than Grade 5. It is the extra-low-interstitial version of Ti-6Al-4V, with oxygen cut from 0.20% to 0.13%, used where more toughness is needed.
• The H grades (2H, 7H and others) keep the same chemistry and only raise the tensile minimum to 58 ksi (400 MPa), mainly for pressure vessels.

A third meaning: strength level

In 18Ni maraging steel the word means something else again. Under MIL-S-46850D, Grades 200, 250, 300 and 350 are the nominal tensile strength in ksi. That is the number in the names 18Ni250, 18Ni300 and 18Ni350. The same specification also has Types I to IV, which are the melting practice. An order has to state both.

One word, three meanings. When you see “Grade”, check which specification it comes from first.