Non-destructive Testing Usages in the Foundry

The casting for a large, closed impeller manufactured by Stainless Foundry & Engineering (SF&E) has a final ship weight of 2,500 pounds. It is a nuclear service part that was produced at a high specification. In order to leave the foundry, the impeller must pass some critical acceptance criteria.

First, the impeller has to meet the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (B&PVC) Section III, Division I, Subsection NB for Class 1 construction. The material is ASTM A487, grade CA6NM, Class A. This is a fine-grained martensitic stainless steel alloy, which is magnetic in nature.

The only way SF&E and its customer can be certain the closed impeller passes the customer’s purchasing requirements is to run a series of non-destructive tests (NDTs). This means the quality must be confirmed without causing destruction or damage to the part.

NDT is primarily used to evaluate and safeguard the integrity of castings. Many different techniques can be administered, from using a method as basic as visual testing to a method as complicated as ultrasonic testing – all escalate the level of precision.

The requirements placed on castings at the design phase of a project specify the method and acceptance criteria of the casting. In the case of the closed impeller casting, the customer required testing and certification for 100% Radiographic Testing (RT), Magnetic Test (MT), Visual Testing (VT), or visual inspection.

The technicians that perform the NDT in the foundry industry are specially trained personnel. Formal training along with on-the–job history or experience is vital to success.

Personnel Techniques

There are three levels of non-destructive certification qualification with entry level being a Level 1, more experienced as Level II, and most qualified at Level III. The organized certification body for NDT personnel is the American Society of Nondestructive Testing (ASNT). A few different programs to certify personnel are as follows:

• ASNT ACCP – American Central Certification Program is a portable certification (travels from one employer to another) for administration of certification for Levels I, II and III. ASNT is the administration body of these tests. This is the most universal certification method.

• ASNT SNT-TC-1A – Is company directed and generally led by an onsite NDT Level III in the technique being certified to. This is the method that SF&E uses currently. It is the standard of the industry for training personnel to the most germane situations (casting and weld inspections) that they need to perform.

• ASNT CP-189 – Is a consensus standard that follows all of the guidelines of SNT-TC-1A but addresses mandatory training elements and testing questions. It is used where a broader NDT approach is needed. In the foundry industry, the SNT-TC-1A program offers you the maximum amount of flexibility allowing you to laser in on what is needed to ensure quality.

Surface Techniques

Surface techniques are used to look for flaws or the lack of flaws in demonstrating conformance to specification when evaluating castings or welds. These techniques are limited to visibly accessible surfaces.

VT

Visual Testing (VT), or visual inspection, is the oldest, simplest and most widely used NDT method available. VT can be applied to inspect castings and machined components, as well as welds, and is used in all different types of industries. VT can be performed with mirror, artificial lighting and, if required, magnification. Most material specifications for castings have minimal quality standards for acceptable and non-acceptable surface defects and conditions.

PT

Penetrant Testing (PT) is a very sensitive technique for detecting surface-breaking discontinuities and through-thickness leak paths in nonporous materials, like castings. It is one of the basic NDT methods with numerous applications across the foundry industry. PT is a very portable technique that can be performed in remote locations to look for cracks, porosity or other harmful surface indications.

MPI/MT

Magnetic Particle Inspection (MPI), also sometimes referred to as Magnetic Test (MT) is an NDT method for the detection of surface and sub-surface discontinuities in ferrous materials. Materials must be able to have an attraction to a magnetic signature in order to be effective, so non-magnetic materials do not have the capacity for this test method. During MPI, visible ferrous particles are sprayed on the test surface and will align themselves to highlight the defect. This technique is used with dry powder or a fluorescent particle solution.

Volumetric Techniques

A volumetric technique evaluates the casting or machined component below the surface where voids such as gas and shrinkage defects may commonly be found. Volumetric evaluation accurately catalogs where internal flaws are if present within the casting.

RT

Radiographic Testing (RT) is a method which uses either X-rays or gamma rays to examine the internal structure of manufactured components and identify any flaws or defects. At SF&E, our providers employ the use of X-rays in both traditional with silver nitrate processing or digital radiography, where images are saved on computer databases. RT is hands down the best evaluation method to find sub-surface defects economically and accurately.

UT

Ultrasonic testing (UT) is a non-destructive test method that utilizes sound waves to detect cracks and defects in parts and materials. It can also be used to determine a material’s thickness, such as measuring the wall thickness of a pipe. Onsite at SF&E, we perform thickness testing where the microstructure of the material allows. Fine grained materials, such as cast 400 series stainless steel, do well with this method, whereas large grained materials like cast 300 series stainless steel materials (CF8M, CF3, CN7M, etc.) as well as Nickel based alloys do poorly. UT is used in conjunction with RT, commonly, as we detail in the continuation of the impeller case study below.

NDT Detail on Closed Impeller

The production of the closed impeller introduced at the beginning of this article required heat treatment prior to welding or NDT. After heat treatment, we performed the following NDT in this order:

• VT 100% inspection marking locations for excavation and grinding that don’t meet the visual callout acceptance of MSS SP-55.
• RT the casting 100%.
• UT of the unacceptable view locations. The UT is more accurate than the RT previously performed and allows for a more accurate excavation locations and a more surgical approach.
• PT performed in key locations for information only.
• After all RT, VT, PT and UT was performed, all welding was completed.
• Post-weld heat treatment.
• Final VT and MT performed at 100%, the impeller passed.
• The casting was now ready for certification.

This casting is very unique in that it actually employed all five (VT, RT, MT, PT and UT) NDT techniques that we discussed previously. This part was one in a series. All parts were shipped with minimal rework. RT was conducted on three castings. Of the120 X-ray views shot, one view identified a failing on only one casting, which is extremely impressive.

If you have questions to what is the best NDT technique to administer, feel free to reach out to sales@stainlessfoundry.com to begin the collaboration.