heat treatment - hardenabiliity, API 6A
HEAT TREATMENT AND HARDENABILITY
1. SCOPE
This Engineering Standard provides guidelines for selection of materials based on their hardenability. This standard also provides a guide to pre-machining prior to heat treatment.
2. BACKGROUND
Steel is strengthened through heat treatment. During heat treatment, the steel goes through several phase changes. The desired product during the quenching process
is martensite which is significantly harder and stronger than pearlite and ferrite. The cooling rate during the quenching process determines if the austenite will transform to martensite. This cooling rate indicates the hardenability.
Steel with low hardenability requires a faster cooling rate. When steel is cooled faster, the yield strength of the material is less homogeneous compared to one that is cooled over a longer period.
3. REQUIREMENTS
Refer to Table 1.
Column 1: Maximum allowable bar diameter that will maintain minimum yield strength throughout a round bar forging.
Column 2: Maximum wall thickness that will achieve the required yield strength. Column 3 and 4: Cheapest material that will achieve the required yield strength of
60ksi and 75ksi respectively. All other materials BELOW the row will meet the requirement.
For example, a 10” diameter round bar, AISI 4130, 75 ksi material is specified.
From Table 1, 4130 has hardenability up to 1.5” wall thickness. Therefore the
hardness distribution is as shown in Figure 1.
Figure 1: Example of hardness distribution for AISI 4130, 75ksi material
Bar Diameter Wall
Thickness 60 ksi 75 ksi
1 0.5 1040 4130
2 1 1040 4130
3 1.5 4130 4130
4 2 4130 4140
5 2.5 4130 4140
6 3 4130 4140
7 3.5 4130 2-1/4 Cr-1Mo Restricted
Carbon
8 4 4130 2-1/4 Cr-1Mo Restricted
Carbon
9 4.5 4130 2-1/4 Cr-1Mo Restricted
Carbon
10 5 4130 2-1/4 Cr-1Mo Restricted
Carbon
11 5.5 4130 2-1/4 Cr-1Mo Restricted
Carbon
12 6 4130 2-1/4 Cr-1Mo Restricted
Carbon
13 6.5 4140 2-1/4 Cr-1Mo Restricted
Carbon
14 7 4140 2-1/4 Cr-1Mo Restricted
Carbon
15 7.5 4140 2-1/4 Cr-1Mo Restricted
Carbon
16 8 4140 2-1/4 Cr-1Mo Restricted
Carbon
17 8.5 4140 2-1/4 Cr-1Mo Restricted
Carbon
18 9 4140 2-1/4 Cr-1Mo Restricted
Carbon
19 9.5 4140 2-1/4 Cr-1Mo Restricted
Carbon
20 10 4140 2-1/4 Cr-1Mo Restricted
Carbon
21 10.5 4140 410, F6NM, 718
22 11 4140 410, F6NM, 718
23 11.5 4140 410, F6NM, 718
24 12 4140 410, F6NM, 718
Table 1: Material Hardenability Table
PRE-MACHINING
Advantages
As the size of the material increase, the hardness at the core drops dramatically. In order to obtain a material with homogeneous yield strength throughout, the material needs to be pre-machined close to the final dimensions. For through bores, the minimum bore diameter must be at least 2” to allow the quench media to flow through the bore and produce an effective quench.
Figure 2 shows the effect of machining a through bore. If no through bore is machined as shown in the left figure, the hardness is not achieved in the center.
However, if a 2” bore is machined in the bar BEFORE heat treatment, the entire material is maintained at above 75ksi yield strength.
Figure 3 shows the exterior of the flange pre-machined. If the exterior is not machined down to 6” as shown, a portion of the forging will not be able to achieve above 75ksi yield strength.
Figure 2: Effect of pre-machining a through bore
Figure 3: Effect of pre-machining
Disadvantages
4.1. Pre-machining adds cost and time. In some cases, it may be cheaper to use a more expensive material.
4.2. Pre-machining can lead to quench cracking unless care is taken to avoid stress riser points. The following considerations should be implemented when designing forging drawings
4.2.1. Avoid sharp corners. Put at least a ½” fillet radius at all
Figure 4: Avoiding quench cracking #1
4.2.2. Avoid intersecting bores
Figure 5: Avoiding quench cracking #2
4.2.3. Avoid multiple bores. However, in cases such as splitter wellheads, dual bore may be required to maintain the hardness level. For example, a 36” starter head may require 2x 8” bores to maintain the hardness level.
Figure 6: Avoiding quench cracking #3
4.2.4. Avoid section changes in bore
Figure 7: Avoiding quench cracking #4
5. DESIGN CONSIDERATIONS
5.1. Not every component requires the yield strength to be at or above the specification throughout the entire body. For example
5.1.1. Some of the thicker API blind flanges may not have the specified yield strength at the core.
5.1.2. In most cases, sealing areas for elastomers need not be of high yield strength as elastomer is always softer than steel used in our industry.
5.1.3. Design bearing stress may be so much lower than the allowable that it is acceptable to have the localized yield strength lower than that in the material specification.
The Engineer needs to exercise discretion and decide if the particular component or region needs to have the specified yield strength.
5.2. Certain areas are required to have the specified yield strength due to API requirement. For example, hardness testing on end connection faces is required for PSL 3 equipment under API 6A, 19th Edition, para 7.4.2.3.3. The hardness must meet the minimum value specified in para 7.4.2.1.3c.
5.3. When specifying the hardness test location on the machine drawing, do not select an area where the hardness value will most probably fall below the material specification and is not required to meet the specified yield stress.
This will create unnecessary NCR and paperwork.
5.4. All forging drawings shall include the following note: “ALL DIMENSIONS SHOWN ARE REQUIRED PRIOR TO HEAT TREATMENT”.
5.5. Considerations should be given to specify a higher hardness range in areas where the hardness may drop after welding, stress relief or any machining.
For example, if a MS has a hardness range of 197 to 237 HB for a flange, the weld neck area should specify 207 to 237 HB to allow drop in hardness
values.
