API 510 Chapter 12

API 510 Chapter 12 – Impact Testing

12.1 Avoiding brittle fracture In any item of structural or pressure equipment there is a need to avoid the occurrence of brittle fracture. As we saw in API 571, brittle fracture is a catastrophic failure mechanism caused by the combination of low temperature and a material that has a low resistance to crack propagation at these temperatures. Under these conditions a material is described as having low toughness (or impact strength) – i.e. it is brittle. Impact strength is measured using a Charpy or Izod test in which a machined specimen is impacted by a swinging hammer. Figure 12.1 shows the situation.

Figure 12.1 The Charpy impact toughness test
Figure 12.1 The Charpy impact toughness test

ASME design codes take a simplified pragmatic view of the avoidance of brittle fracture. Their view is that there are two levels to the checks required:

First, there is a simple set of rules to determine if a material (and its design temperatures) actually needs impact testing or whether it can be assumed to be tough enough without being tested.

Second, if it fails the first set of criteria and does need to be tested, what Charpy (Joules) or Izod (ft-lb) results need to be achieved for the material to be considered codecompliant.

Historically, you can expect a couple of questions in the API 510 exam relating to each of these criteria. The first criterion is a little more difficult to understand as there are several parts to it, and the code is not that easy to interpret on a casual reading. The second part is easier and just involves reading figures from tables, once you know where to find them. We will look at this now in UCS-66.

12.2 Impact exemption UCS-66

The main exam questions on this subject come from the tables and charts of UCS-66. Strictly, there are some opportunities for overall impact test exemptions that may apply before UCS-66 is even considered – these are tucked away in a totally separate part of the code: UG-20. Don’t worry too much about these UG-20 requirements. They appear rarely, if at all, as exam questions, because they would divert attention away from UCS-66, which is where the impact strength questions usually come from.

In concept, UCS-66 is straightforward – the steps are as follows (see Fig. 12.2):

Step 1. For a given material determine, from figure UCS-66, whether it is covered by material curve A, B, C or D. Simply read this off the table, being careful to read the notes at the bottom of the table. In particular, notice that

Figure 12.2a1 The UCS-66 steps. Courtesy of ASME (continues on next page)
Figure 12.2a1 The UCS-66 steps. Courtesy of ASME (continues on next page)

 

Figure 12.2a2 The UCS-66 steps. Courtesy of ASME
Figure 12.2a2 The UCS-66 steps. Courtesy of ASME

 

Figure 12.2b The UCS-66 steps. Courtesy of ASME
Figure 12.2b The UCS-66 steps. Courtesy of ASME

curve A provides a default for any relevant materials not listed in curves B, C or D. Note also how a material that has been normalized may be in a different group to the same material that is non-normalized. This is because normalizing affects the grain structure, and hence the brittle fracture properties.

Step 2. Determine the nominal thickness of the material. This is normally given in the exam question.

Step 3. In figure UCS-66 (for US units) or figure UCS-66M (for SI units), check the material thickness on the lower (horizontal) axis. Then read up the graph until you reach

Figure 12.2b The UCS-66 steps. Courtesy of ASME
Figure 12.2b The UCS-66 steps. Courtesy of ASME

the relevant curve A, B, C or D and read off the corresponding temperature on the vertical axis. Figure 12.2 (a) and (b) shows the procedure.

Step 4. Now the important part – the reading you just obtained on the vertical axis is the minimum temperature at which the component can be used (i.e. designed to be used) without requiring impact tests to check its resistance to brittle fracture. This design temperature is referred to as the minimum design metal temperature (MDMT) and is shown on the vessel nameplate. If you are confused by this, just follow these two rules: 

  • If the required MDMT (i.e. the lowest temperature that you want the vessel to operate at) is higher than the temperature on the vertical axis of figure UCS-66, then impact tests are not required (because the material is not brittle at that temperature).
  • Conversely, if the required MDMT is lower than the temperature on the vertical axis of figure UCS-66, then impact tests are required, to see if the material has sufficient toughness at that temperature.

Step 5. Check the figure UCS-66.1 ‘low stress ratio temperature reduction’. A feature of the ASME VIII-I code is that a material is considered less susceptible to brittle facture at a set temperature if the stress on the component is low. Technically, this is probably a disputable point, but the ASME codes have used it successfully for many years. The stress ratio is defined simply as the amount of stress a component is under compared to the allowable stress that the code allows for the material. It varies from 0 to 1.0, i.e. 0 % to 100 %, and in an exam question is normally given.

Figure UCS-66.1 and Fig. 12.2(c) show how the stress ratio reduction is used. This time, enter the graph on the vertical axis at the given stress ratio, move across to the curve and then read off the coincident temperature on the horizontal axis. This figure is the temperature reduction that can be subtracted from the previous temperature location on the vertical axis of figure UCS-66.

Step 6. Check the UCS-68 (c) ‘voluntary heat treatment temperature reduction’. This is the final potential reduction allowed to the MDMT. Clause UCS-68 (c) (a few pages forward in the code) says that if a vessel is given voluntary heat treatment when it is specifically not required by the code (i.e. because the material is too thin or whatever), then a further 30 °F reduction may be applied to the MDMT temperature point identified on the original UCS-66 vertical axis. Note that this is in addition to any reduction available from the low-stress scenario.

Final step – general ‘capping’ conditions. Hidden in the body of the UCS-66 text are a couple of important ‘capping’ requirements. These occasionally arise in the API exam. The most important one is clause UCS-66 (b)(2). This is there to ensure that the allowable reductions to the impact exemption temperatures don’t go too far. Effectively it ‘caps’ the exemption temperature at 55 °F for all materials. Note, however, the two fairly peripheral exceptions to this when the 55 °F cap can be overridden. These are:

  • When the stress ratio is less than or equal to 0.35 (i.e. the shaded area of figure UCS-66.1). This is set out in UCS-66 (b)(3) and reinforces the ASME code view that components under low stress are unlikely to fail by brittle fracture.
  • When the voluntary heat treatment of UCS-68 (c) has been done and the material is group P1.

The exam questions Historically the API 510 exam questions on impact test exemption are pretty simple. They rarely stray outside the boundary of figure UCS-66 itself. The allowable reduction for low stress ratio and voluntary heat treatment are in the exam syllabus, but don’t appear in the exam very often.

Now try these familiarization questions on impact test exemption.

12.3 ASME VIII section UCS-66: impact test exemption familiarization questions

1.

Q1. ASME VIII section UCS-66
A stationary vessel is made from a 3 in thick SA516 GR70 plate that has been normalized. The MDMT is 30 °F at 500 psig. Does this material require impact testing?

 
 
 
 

2.

Q2. ASME VIII section UCS-66
A vessel constructed of ‘curve B’ material is to be patch-plated with a fillet welded patch of the same material as the shell. The stress ratio is calculated as 0.64. The patch and vessel are 0.622 in thick with zero corrosion allowance. The MDMT is -15 °F. From the information given, does the repair require impact testing of the repair procedure?

 
 
 
 

3.

Q3. ASME VIII section UCS-66
A 1.125 in the thick lap-welded patch is of SA-515 Gr 70 P1 material. The vessel nameplate shows MDMT as 50 °F ‘HT’, denoting that the patch has been voluntarily heat-treated. The stress ratio is 1. From the information given, does the repair require impact testing of the repair procedure?

 
 
 
 

Click Here To Read Next API 510 Chapter 13 – Introduction to Welding/API 577 

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