Creep-resistant-steels:

how to select and weld.

SOLUTIONS with Effective, Practical Advice

Creep-resistant-steels are those providing useful properties at service temperatures exceeding those admitted for carbon steels.

According to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, normal carbon steels are considered suitable for service at temperatures not exceeding 345 0C (650 0F).

At this maximum temperature, per the above Code, the maximum safe design stress shall not exceed 25% of the minimum specified Ultimate Tensile Strength of the steel selected.

ASME Specifications define the types and the properties of several steels, like the following SA 515, SA 516, SA 302, SA 202, and SA 537 for plates, each of them further defined according to Grades or Classes.

It is well known that the mechanical properties of materials are modified as a function of temperature. Specifically the Ultimate Tensile Stress and the elastic limit decrease in various degree with increasing temperature depending on the type of steel.

For service at elevated temperature the above properties are not the governing factors. In fact steels may be subject to excessive plastic deformation for stresses well below those limits if applied for long enough time.

For use at elevated temperatures, besides the long time stability and properties of Creep-resistant-steels, one has to consider also the resistance to oxidation and scaling at the service atmosphere as well as the resistance to corrosive attacks.

Furthermore, before finalizing the selection, one has to take into account fabricability, weldability, maximum run-off temperature, expected life of the construction and cost. Particular attention requires service in presence of hydrogen at elevated temperature and pressure.

A short introduction to Heat Resistance was published in Section 2 of Issue 55 of our Practical Welding Letter for March 2008. Click on PWL#055 to see it. In the same Issue, in Section 7, a short article deals with a related subject, Type IV Cracking.

In practice to improve the properties of Creep-resistant-steels one modifies the metals by alloying them with specified small additions of elements like Chromium, Molybdenum, Vanadium, and other materials having definite influence on favorable modifications of metallurgical microstructure.

The accepted criterion for evaluating different Creep-resistant-steels is the stress that, if applied continuously on specified specimens at the given temperatures for one hundred thousand hours (corresponding to slightly more than eleven years), will cause in that time period an elongation
not exceeding 1% of its basic gauge length. This will be the reference for establishing safe design stresses.

Creep-resistant-steels for high temperature service are those suitable for applications that require exposures between 345 and 815 0C (650 and 1500 0F). For each one of the approved materials the usual Code establishes the allowable design stress at service temperature.

The following Table lists a few Creep-resistant-steels usually considered for use in the specified temperature ranges.

Table
Creep Resistant Steels
ASME No. Composition % Temperature Range
Cr Mo C F
SA 204, grade C -- 0.5 430-510 800-950
SA 302, grade B -- 0.5 430-510 800-950
SA 387, grade 12, class 1 1.0 0.5 455-565 850-1050
SA 387, grade 12, class 2 1.0 0.5 345-480 650-900
SA 387, grade 11, class 1 1.25 0.5 455-565 850-1050
SA 387, grade 11, class 2 1.25 0.5 345-480 650-900
SA 387, grade 22, class 1 2.25 1.0 455-595 850-1100
SA 387, grade 22, class 2 2.25 1.0 370-480 700-900
SA 387, grade 5, class 1, class 2 5.0 0.5 480-620 900-1150
SA 387, grade 9 9.0 1.0 510-595 950-1100
SA 387, grade 91 (1) 9.0 1.0 540-650 1000-1200
SA 240 Austenitic (2) (2) (2) 595-815 1100-1500

(1) + Ni, V, Nb, N, Al
(2) - Austenitic Stainless Steels:
Grade 304H 18Cr-8Ni
Grade 316H 16Cr-12Ni-2Mo
Grade 321H 18Cr-10Ni-Ti
Grade 347H 18Cr-10Ni-Nb

Excluding austenitic stainless steels the other Creep-resistant-steels listed above, in particular those containing Chromium and Molybdenum are all martensitic, that is they will harden upon quick cooling from elevated temperatures like those found in welding.

The usual precaution, to avoid cracking as a consequence of shrinking stresses, is to apply preheating before welding to decrease the cooling rate, and then to relieve the stresses as soon as possible after welding especially for thick materials and/or for constrained structures.

Filler metals are generally similar in composition to the base Creep-resistant-steels except if post weld heat treatment cannot be performed. In that case austenitic filler metals may be preferred to provide more ductility in the weld.

While base metals Creep-resistant-steels may be quite acceptable for elevated temperature service, it must be noted that welded joints are a potential source of weaker behavior in creep due to their intrinsic lack of homogeneity and possibly to the presence of weld discontinuities.

The usual practice, as established in codes for nuclear plant components, is to reduce even more the stresses admissible in weldments.

A related subject, Risk Based Inspection, concerning the continuing assessment of safe working conditions of plants designed to operate for decades without failures, was treated in Section 2 of Issue 57 of Practical Welding Letter for May 2008. Click on PWL#057 to see the article.

* * *

Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.

Hardness Testing
made simple Let us remind you that, if you are interested, we offer a no cost subscription to our Practical Welding Letter and a bonus book in pdf format to be made available for download to your computer on the subject of
PRACTICAL HARDNESS TESTING MADE SIMPLE. Click on Subscription.

To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.

Back Home
Processes
Site Map

Cast Iron Welding
Steel Welding
Alloy Steel Welding
Tool Steel Welding
Stainless Steel Welding
Aluminum Welding
Magnesium Welding
Titanium Welding
Copper Welding
Heat Resisting Alloys Welding
Joining Lead Tin Zinc
Refractory Metals
Precious Metals
Welding of Plastics
Joining Galvanized Steels

Important Announcement

See our New Page on Metals Knowledge for assembling at no cost an Encyclopedia Online, a rich collection of valuable information on Metals, from expert Internet sources.


Google
 
Web www.welding-advisers.com

How can you solve
your Welding Problems?

Click on Welding Consultation.

Top

Click on the following image to watch the SBI! TV Show!

SBI TV Show


POWERED BY:

Site
Build It!

Click on this Logo NOW!

Copyright (©) 2008, by
Elia E. Levi and
www.welding-advisers.com
All Rights Reserved