# MINIMUM DESIGN METAL TEMPERATURE (MDMT)

IMPACT TEST

1. In any item of structural or pressure Equipment there is a need to avoid the occurrence of brittle fracture.
2. Impact strength is measured using a Charpy or Izod test.
3. Assume ALL Carbon and Low Allow Steels require impact test, EXCEPT:

a. UG-20

b. UCS-66 (MDMT and Thickness)

c. UHA 51

MINIMUM DESIGN METAL TEMPERATURE

The main exam questions on this subject come from the tables and charts of UCS-66.

1. For a given material, determine from figure UCS-66, whether it is covered by material curve A, B, C or D.

-Be careful to read the notes at the bottom of the table.

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

3. In figure UCS-66, check the material thickness on the lower (horizontal) axis. Then read up the graph until you reach the relevant curve A, B, or D and read off the corresponding temperature on the vertical axis.

2. 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.

3. It varies from 0 to 1.0, i.e 0% to 100%, and in an exam question is normally given.

MDMT EXAMPLE

Vessel made of SA-516 Gr70. are not normalized. Rated 250 psig at 700 F. Wall thickness is 0.5″, with a corrosion allowance of 0.1″. Reduction ratio is 0.85. HT was applied.

1. Determine Curve Letter.
2. Initial MDMT (Fig UCS-66)

a. Material: Curve B

b. Initial MDMT = -7 F

3. MDMT reduction (Fig UCS-66-1)

a. Reduction ratio: 0.85

b. Reduction = 15F

c. New MDMT = Initial MDMT – Reduction = -7 – 15 = -22F

MDMT PWHT REDUCTION UCS-68 (C)

1. Given when vessel has been PWHT, when actually not required by UCS-56.
2. If yes, subtract additional 30 F from the MDMT.
3. From previous example: HT was applied.

a. It was not required by UCS-56

b. Therefore: -22F – 30F = -52F

4. LASTLY, Refer to UCS-66 (b) 2 for lowest temperature limit.

CHARPY IMPACT TEST

UCS-66 (a) speifies that a Charpy Impact Test is always required if:

-the thickness at welded joint is > 4 in.

-MDMT is < 120F

a. Thickness of non-welded part is > 6 in.

b. MDMT is < 120F.

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