🔍 “Struggling to Understand forming of pressure parts and fibre elongation?” 🤔

What is Forming?

Forming refers to any metal working process that involves shaping of metal, such as rolling a shell or making Dish-end through spinning or point bracing. In the context of pressure vessel design, forming is a widely used process that is necessary for creating pressure vessel parts.

Two common forming techniques in the pressure vessel industry: Shell forming and Dish-end forming. Additionally, we will explore what the code says about forming, and whether there are any specific guidelines or regulations that we should be aware of.

Types Of Forming:

There are two types of metal forming: Cold forming and Hot forming. Cold forming involves shaping metal at room temperature or below the recrystallization temperature, while hot forming involves shaping metal at a temperature above the recrystallization temperature. It is important to note that the basic limit of cold forming is not limited to just room temperature. It can occur up to the recrystallization temperature, which is the temperature at which the metal's microstructure changes due to deformation. For carbon steel, this boundary line between cold and hot forming is approximately 705°C. Understanding the difference between cold and hot forming is crucial in selecting the appropriate technique for a given metalworking application.

What is Fiber Elongation?

Imagine, if we are doing forming operation, then there is going to be a fiber elongation, now, what exactly is fiber elongation?

Let’s understand, if we take a plate, and we want to bend it or want to form it. forming is nothing but kind of bending of that plate. So, once you are doing this operation you can feel there will be compression at one side and there will be tension at the other side (See Picture above).  when there is tension on one side, there you can see your fiber will get elongated. It's nothing but the change in length of that fiber. So, outer side fiber will have more length than the neutral one, that is called fiber elongation.

Whenever we do any forming like, add a radius to the plate, such as a crown or knuckle, there will be fiber elongation in these areas. among these two, the sharper the radius, the greater the fiber elongation will be. The code also gives us the formula to calculate fiber elongation. The  formula are for two different parts, like cylinders and also the for heads or double curved formed parts.

Equation For Forming Strain (Plate) (Table UG-79-1):

𝜀𝑓= Extreme fiber elongation

𝑅𝑓= Final mean radius

𝑅o= ??

Code has given us a formula to calculate fiber elongation. This formula is simple and applies to both cylinders and doubly curved formed parts, such as dish ends (crowns and knuckles).

When we start the forming process, we begin with a straight plate that we purchase from the market. We then perform the forming operation, and the plate takes the shape of a cylinder with the desired nominal ID, which becomes the final radius of the forming process. In the formula, we have two variables, Rf and Ro, (see above formula) which represent the final and starting radii, respectively.

How to get Ro?

When a circle is small, (see above image) its section appears curved. However, as the circle becomes bigger, its section appears straight. For instance, the Earth's surface appears flat to us because its radius is much larger than what we can perceive. This is why we see it as a straight line. Essentially, a straight line means the radius is becoming very large, and if it is completely straight, it can be defined as having an infinite value.

Now, Let’s See the Example for,

Example For Forming Strain (Plate):

Considering the Data Given in the Table, and having understood Initial mean radius (∞) and final mean radius (nominal ID/2 + thk/2), we can easily calculate the fiber elongation. For example, let's use the formula with an ID of 2500 and a plate thickness of 10 mm.

𝜀𝑓 =( 50t/𝑅𝑓 )(1 -𝑅𝑓/Ro)

𝜀𝑓 =((50*10)/1255)(1-1255/∞))

=(50*10)/1255)

= 0.4%

The result for (50 x 10 / 1255), with the Ro value being infinity. The resulting fiber elongation percentage is only 0.4%.

Now, let us take one example of doubly curved parts like, Dish end (2:1).

Example of Forming Strain (at Knuckle):

ID = 2500 mm

Nominal Thickness (t) = 12 mm

Knuckle radius = 0.17*ID + t/2 = 431 mm

Double Curvature (e.g.: heads)

𝜀𝑓 =(75t/𝑅𝑓 )(1-𝑅𝑓/Ro)

𝜀𝑓=((75*12)/431)(1-2250/∞))

=((75*12)/431)

= 2.088 % (at knuckle)

What to do with Fiber Elongation?

We have taken a plate, and we have formed a Dish-end and if you see the knuckle there, there will be more elongation and high induced stresses. So, by calculating fiber elongation, we are trying to determine the stresses induced because of forming, these stresses are not because of pressure, this is because of forming operation.

Now few questions will come in our mind:

How to know if the stresses are high?

What to do if the stresses are high?

Do we need to perform PFHT (Post Forming Heat Treatment ) to reduce these stresses?

We need to move to material specific clauses to answer these questions. For different materials with different fiber elongation, specific codes must be followed to determine when to perform PFHT. The relevant clauses, for carbon steel are found in UCS-79(d). For nonferrous alloys, one must refer to UG79(a), for high Alloy Steel UHA-44(a) is to be referred. and for ferritic steel UHT-79(a).

•   UCS-79(d) - Carbon and low Alloy Steel

•   UNF-79(a) - Non-Ferrous Alloys

•   UHA-44(a) - High Alloy Steel

•   UHT-79(a) - Ferritic Steel

UG-79 gives us the formula to calculate fiber elongation, but it is crucial to follow the clause for a specific material to decide if the PFHT is required (Induced Stresses are high) or not (Induced Stresses are low).

To relieve undue stresses, a heat treatment process called Post Forming Heat Treatment (PFHT) is often applied. It is important to know when to apply PFHT for a given material.

How to decide if PFHT is required?

For carbon and Low Alloy steel, we need to refer to the UCS-79(d). As per UCS-79(d), If the fibre elongation percentage exceeds 5%, we need to perform PFHT.

However, there are some exceptions to this rule,

For P number 1, group number 1 and 2 materials, if all following conditions are met:

a)  no lethal substance stored,

b)  no impact testing requirement,

c)   nominal thickness less than 16 mm,

d)   reduction in nominal thickness less than 10%, and

e)   forming temperature not in the range of 120°C to 480°C

then, even if the fiber elongation is up to 40%, heat treatment is not required. If the fiber elongation exceeds 40%, then only the heat treatment will be necessary.

Now, if PFHT is required, then what cycle to be followed for UCS materials?

UCS-79(d) talks about that UCS-56, which is applicable for post weld heat treatment same cycle we must follow for post forming heat treatment. The reason for using the same cycle for PFHT is that both processes involve relieving stresses in the material. That is the reason some time we call it stress relieving cycle.

For High Alloy Steels:

We need to follow Table UHA-44. How to read that table?

Take an example of Grade 304, if design temperature is exceeding 580 but less than 675 and forming strain calculated as per UG-79 exceeds 20%, we need to perform heat treatment (solution annealing) at minimum 1040⁰C with holding time calculated based on 20 min / 25 mm.

Thank you..

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