Alloy Steel Hardenability for Wellhead Equipments

Figure 1 – Hardenability on Alloy Steel
(Adapter from [2] page 1272 and [5] page 391 edited)

Wellhead equipment (and most of equipment made from alloy steel) shall uniquely exhibit certain mechanical properties. As along the way to reach those properties, such as: hardness, mechanical strength, etc, alloy steel shall need to undergo heat treatment process.

This is where hardenability plays some role to define a degree of steel that could be hardened by forming martensite microstructure as a result of heat treatment, especially quenching. Hardenability is a qualitative measure of the rate at which hardness value drops off with distance into the interior of a specimen from a diminish martensite content. Alloy steel considered high hardenability is the steel that hardens, or form martensite, not only at the surface but to a large degree throughout the entire interior. As being mention earlier, alloy steel has unique characteristics depends on heat treatment which determine their hardenability. Hardenability depends on:

1. Carbon content ( > 0.35% susceptible to quench crack)
2. Alloy element
3. Grain size (↑ size≈↑ hardenability)
4. Quenching medium (cooling rate)

ASTM defined standard to measure hardenability on ASTM A255.This standard defines two methods, Grossman method or Jominy test. Grossman method determine hardenability with chemical composition calculation to obtain critical diameter (DI), diameter of a given steel that would produce 50% martensite at the center when quenched in a bath of quenchant with quenching severity consideration. Some literature mentioned this critical diameter as Limited Ruling Section (LRS). Nowadays, this method was not popular as compare to the other one due to time consuming.

Jominy test is done by heating a piece of solid specimen in cast or wrought product into its austenite temperature and quench. Once the quenching process done, hardness value being taken at certain distance. Jominy test result being reported in a hardenability curve which shows decreasing hardness value from the edge point quench process start. ASTM also has documented several hardenability curve of steel composition number started with suffix letter ‘H’ in ASTM A304.

AISI 4130 is one of low alloy steel which is widely used for wellhead equipment. In this post, I try to examine how to translate jominy hardenability curve to obtain hardness value distribution of 3” solid bar from surface to solid bar center. ASTM A304 documented hardenability curve with some of hardness information on surface, 3/4 radius from center, and the center of certain bar diameter. The result also dictates hardness value with two different quenchant, mild water and mild oil. I put it in below self explanatory picture to show how to translate Jominy curve to other curve which I find it more meaningful rather than the former.

Figure 2 – How to Translate Hardenability Curve into Hardness Distribution
(Adapter from [2] page 12 edited)

This information can give us an idea how hardness value drops from surface of 3” bar diameter to the center. Instead, ASTM also provide more information by providing hardness value in two quenching medium. This additional information also help us to visualize how different hardness drop off when it comes to exposed on different medium.

Figure 3 – AISI 4130 Hardness Distribution with Water and Oil Quenching Medium

The graph shows us how hardness value abruptly changes when the steel being quench by water. Every steel manufacture has their own common practice about how to cook their steel and do heat treatment. Some manufacture sometimes conduct non-destructive test (magnetic particle inspection) on its surface to ensure there’s no crack developed during quenching.

Instead of these comparisons, we can compare how different between one steel number to the other. In this example I took 8630 as comparator for its resemblance to 4130 in terms of chemical composition and it’s widely used in subsea wellhead application. 8630 is a kind of alloy steel which contains nickel as an additive alloy element for increasing toughness, better corrosion resistance (and more costly as well). Based on this graph comparison, we can see how 8630 exhibit similar characteristic with 4130 with quenching by oil.

Figure 4 – Hardness Distribution of AISI 4130 and 8630 Comparison with Oil Quenchant

However, as the attempt to determine hardness distribution along radial location, all these data shall only give us qualitative idea how close/different between one steel to another and for sure, the real value of hardness distribution really depend on how the steel manufacture ‘cook’ their steel.

Reference:

1. ASTM A255, 2010.Standard Test Methods for Determining Hardenability of Steel. ASTM International.
2. ASTM A304, 2011. Standard Specification for Carbon and Alloy Steel Bars Subject to End-Quench Hardenability Requirements. ASTM International.
3. ASM Handbook, Volume 1. 10^th Edition. Properties and Selection: Irons Steels and High Performance Alloys. ASM International
4. Bellarby, Jonathan, 2009. Well Completion Design. SPE NACE International and TRACS International Consultancy Ltd. Aberdeen, UK. p. 438.
5. Callister, William D, 2007. 7^th Edition. Material Science and Engineering: An Introduction. John Wiley and Sons, Inc, USA. Chapter 11
6. http://www.keytometals.com/Articles/Art146.htm

Advertisements