Casting Quality Factor (Ec) for B31.3

What is the Casting Quality Factor ($E_c$)?

The casting process, which involves pouring molten metal into a mold, can introduce internal discontinuities such as porosity, shrinkage, or inclusions. While modern casting is a highly controlled process, these potential imperfections must be accounted for in a conservative design. The casting quality factor ($E_c$) is the mechanism B31.3 uses to do this.

Similar to the weld joint factor, $E_c$ is a de-rating factor applied to the basic allowable stress ($S$) in the pressure design formulas. Its value typically ranges from 0.80 to 1.0. A factor of 1.0 implies the highest quality, while a factor of 0.80 represents a standard-quality casting that has undergone basic visual inspection. A lower $E_c$ requires a thicker wall for the component to safely contain the same pressure.

The following reading offers a concise overview of quality factors, including $E_c$.

Determining the Basic Casting Quality Factor

The first step in applying $E_c$ is to determine the basic factor for a given material. This value is found in Table A-1A of ASME B31.3.

Let's examine a few entries in this table:

• Ductile Iron (A395): The $E_c$ value is listed as 0.80.
• Carbon Steel (A216): The basic factor is also 0.80.
• Gray Iron (A126): Here, the $E_c$ value is 1.00.

You might wonder why gray iron receives a perfect factor of 1.00 right away, while higher-strength carbon steel starts at 0.80. The rationale is explained in para. 302.3.3(b) of the code.

As the description in the image clarifies:

1. Gray and malleable iron castings are assigned an $E_c$ of 1.0 because their fundamental allowable stress basis is already very conservative, providing a sufficient safety margin without further de-rating.
2. Most other metal castings start with an $E_c$ of 0.8. This assumes the casting conforms to its material specification and has passed a visual examination as required by MSS SP-55.

Therefore, for any cast component, your starting point is Table A-1A. For most common steel alloys, you will begin with an $E_c$ of 0.80.

Increasing the Casting Quality Factor with Supplemental Examination

Just as with the weld joint factor $E_j$, you can "earn" a higher casting quality factor by specifying additional Non-Destructive Examination (NDE) beyond the basic requirements. Given your background in quality and inspection, this principle directly connects the value of NDE to design optimization. Performing more rigorous inspection provides greater confidence in the component's integrity, which the code rewards with a less conservative quality factor.

The rules for increasing the casting quality factor are detailed in Table 302.3.3C of ASME B31.3 (not shown here, but referenced in the code). This table outlines a matrix of examinations—such as magnetic particle (MT), liquid penetrant (PT), ultrasonic (UT), and radiographic (RT) testing—that can be applied to different "examination zones" of a casting. By satisfying the requirements of a specific quality level in this table, the factor $E_c$ can be increased, often to 0.85, 0.90, or even 1.00.

This creates an important engineering trade-off. The designer must weigh the cost of performing extensive NDE against the potential benefits of a higher $E_c$, which could allow for a less thick, lighter, and potentially less expensive component, or provide a higher pressure rating for a standard component.

Let's look at a practical application of this principle.

As the example shows, by specifying full radiography and magnetic particle testing on all surfaces, the designer can justify using an $E_c$ of 1.00 for this A351 casting, even though its basic factor from Table A-1A would have been 0.80.

Worked Examples

Let's put this into practice with two simple scenarios.

Example 1: Standard Carbon Steel Cast Valve

• Question: A piping specification calls for a standard cast carbon steel gate valve manufactured to ASTM A216 Grade WCB. No supplemental examination is specified. What is the correct casting quality factor, $E_c$?
• Solution:
  1. This is a standard cast component with no extra NDE. We need the basic factor.
  2. Refer to Table A-1A (the image Table A-1A Basic Casting Quality Factors).
  3. Find the row for Carbon Steel, Spec. No. A216.
  4. The corresponding $E_c$ is 0.80.

Example 2: Ductile Iron Pump Casing

• Question: The casing for a pump is made from ductile iron conforming to ASTM A395. What is the $E_c$?
• Solution:
  1. Again, this is a direct lookup for a basic factor.
  2. Refer to Table A-1A.
  3. Find the row for Ductile and ferritic ductile iron castings, Spec. No. A395.
  4. The corresponding $E_c$ is 0.80.

Conclusion

You have now mastered the application of the final primary quality factor, $E_c$. You can see the clear parallel between how B31.3 treats welded pipe and cast components, applying a specific factor to adjust the material's allowable stress based on its manufacturing method and the level of quality verification performed.

Here are the key takeaways from this lesson:

• The casting quality factor ($E_c$) is a de-rating factor applied to the allowable stress of cast components to account for potential internal discontinuities.
• The basic $E_c$ for most steel castings is 0.80, assuming compliance with the material specification and visual examination. This is found in Table A-1A.
• Some materials, like gray and malleable iron, have a basic $E_c$ of 1.0 due to their inherently conservative allowable stress basis.
• The $E_c$ can be increased up to 1.00 by performing supplemental NDE (RT, MT, etc.) in accordance with the rules in para. 302.3.3 and Table 302.3.3C of the code.

In this module, we have assembled the essential ingredients for pressure design: design conditions (pressure and temperature), the basic allowable stress ($S$), the weld joint quality factor ($E_j$), and now the casting quality factor ($E_c$).

In our next lesson, we will move into the next module and finally begin using these components. We will dive into the core pressure design calculation: determining the minimum required wall thickness for a straight pipe under internal pressure, putting all this foundational knowledge into direct practice.