Metal Casting Inspection Methods: What They Tell Us

Metal casting is precise and process orientated. With countless variables potentially affecting the result, casting inspection procedures must be rigorous.  Inspection methods can vary between casting type, and different inspection methods are often used for a single casting.  More than anything else, metal casting inspections provide customers with confidence that they received a quality casting.  Before describing the various testing and inspection procedures, we will first focus on different types of metal used for castings.

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Types of Metals for Castings

While any solid metal can be melted and cast, ferrous and non-ferrous alloys are the two main metal types used to create castings.  The simple difference is that ferrous alloys include iron and non-ferrous alloys do not include iron.  Ferrous and non-ferrous alloys also have unique properties.

Prominent examples of ferrous alloys are steel, carbon steel, and cast iron.  These are known for strength and durability.  Non-ferrous alloys include bronze, copper, aluminum, and lead.  Non-ferrous alloys are more malleable compared to ferrous alloys.  Since non-ferrous metals are not iron based, they have high resistance to rust and corrosion.

Methods to Test Casting Quality

There are five primary methods to confirming the quality of a casting,

1. Chemical composition
2. Casting finish
3. Dimensional analysis
4. Mechanical properties
5. Casting soundness

MetalTek also offers metallographic testing, which is an analysis of the microstructure of the material.  This helps determine the reliability of the casting and why a cast material may have failed.  Dense materials are often harder to inspect visually and require some of the more advanced methods.

Chemical Composition

Chemistry inspection is likely the most common casting quality test.  A master melter takes raw elements or pre-alloy materials and mixes them into a charge.  Conceptually, this process is like following a baking recipe.  Adjustments to the charge can be made. The pour is then made into a special mold, or an actual pour is made.  The customer determines whether the chemistry test is performed via a sample or on the final pour.

Chemistry inspection performed at MetalTek&#;s Carondelet Division

Casting Finish

Casting finish inspection is conducted after casting to ensure that it meets customer requirements.  The casting inspection procedure for determining casting finish is often subjective.  Tests are done visually with panels increasing in surface roughness.  There is no definitive tool to evaluate surface roughness.

Dimensional Analysis

While dimensional analysis can be as simple as a visual measurement, customers can also have a Coordinate Measuring Machine (CMM) measurement taken.  This machine is a high-precision measurement tool, commonly used for high specification applications (e.g., valves, jet engines, pump bodies) and high-price tag projects. CMM is by far the most accurate measurement available.

Coordinate measuring machine (CMM) inspector at MetalTek&#;s Wisconsin Centrifugal Division reviews results of a casting scan

Mechanical Properties

This casting quality inspection measures whether the casting meets specified mechanical requirements.  Mechanical tests check physical and mechanical properties including hardness, impact, and service load testing. This type of inspection provides a numerical value and is usually non-destructive.  The hardness numbers are related to metal alloy properties such as wear resistance and machinability.

Casting Soundness

Determining casting soundness is composed of destructive and non-destructive tests. Many internal and surface defects cannot be determined via regular inspection methods.  The &#;invisible&#; flaws can be detected through several different non-destructive or destructive tests.  These tests are how a final product is certified.

How Do Non-Destructive Testing and Destructive Testing Relate to Casting Inspection?

Destructive tests generally involve cutting the casting in half and inspecting for cracks or voids. This renders the casting unusable and is only be performed at the request of the customer.  Destructive tests are generally performed as a proof of copy or test for increased specification projects.

Non-destructive tests (NDT) include X-ray, ultrasound, dimensional, visual, leak, and dye penetrant.  Many of these techniques are used in tandem and depend on the requirements provided by the customer.  No one test is better or worse, but rather depends on the type of casting.  The benefit of a non-destructive test is that castings can still be used after testing is performed.

Liquid Penetrant Inspection (LPI)

Common Casting Defects Found During Inspection

What defects are found during inspection? Even slight process variances can produce critical failures.  Casting defects can come from process variation or process design. Process variables include temperature, speed of pour, and more.  The design process is the casting from mixing to pouring.  No matter how well designed the project is and how well variables are controlled, defects can occur.  The most common defects are voids, casting surface, dimensions, and chemistry errors.

Voids

During the cooling phase around 7% of casting volume is lost, which often produces voids.  Voids are a cause of cracking, but do not necessarily produce them.  The size and pervasiveness will help in the determination of a failure.

Casting Surface

The casting surface that the mold is poured into needs to be spotless or other non-metallic inclusions or discontinuities can quickly enter.  These pieces can freeze into or onto the mold and negatively affect the pour.

Dimension Defects

Dimension defects are like a barber performing a haircut.  A little off is not bad, but you cannot add it back.  Castings shrink during the freezing, but hitting exact specifications requires attention to the finest details of measurements.

Chemistry Errors

Chemistry error is little more than just the mold recipe.  The speed of the pour can change the outcome, as well as other room temperature variations.  However, the right mix of metals is critical above all else.

What Happens If a Part Fails Inspection?

If a casting fails inspection, it is immediately segregated.  Sometimes a casting can be corrected or remediated.  Heat treatment can help mechanical failures, and a re-melt can help with chemistry errors.  Customer specifications again determine what is allowed to be corrected.  Simply starting over is sometimes the best and most cost-effective solution.  At MetalTek the focus is customer specifications and success, so a casting is redone if that is the best option.

Who Performs Metal Casting Inspections?

At MetalTek, every employee serves as an inspector in the sense that casting is a multi-step process and every person involved checks their work before moving it forward. However, there is an additional layer of inspection performed by certified independent inspectors.  These inspectors are present at each of MetalTek&#;s five plants for each process.  New inspectors train onsite and will have to pass a practical exam before ever being on their own.  MetalTek&#;s inspectors are certified to the highest degrees. Certifications include Nadcap, ISO, PED, and NORSOK. In-house testing is most common.

MetalTek employee performs a surface inspection

The casting process and inspection procedures at MetalTek are comprehensive and backed by a customer-driven focus. There is no one solution to testing a cast, but often a conglomeration of testing methods. Integrity is at the base of the brand. It is easy to pass audits and deliver excellence when you do things the right way all the time.

Contact us to learn more.

Coatings shrink as they cure, resulting in low film thickness over sharp edges and welds, which is a classic reason why coatings fail.

Repairing defects is normally not part of an abrasive blaster&#;s job description, but when you are charged with prepping the surface it&#;s imperative the coating adheres properly, and surface defects can prevent that from happening. The abrasive blaster is the only person on the job who looks at the entirety of the steel surface, which makes you the last line of defence for finding defects.

Finding and reporting defects is not only essential but welcome. Providing this additional service is an effective way to make an impression and distinguish yourself from the competition. Before you can accurately and effectively identify defects, you need to know what to look for.

 

Three kinds of steel surface defects

ISO -3, &#;Preparation of steel substrates before application of paints and related products&#;, sorts defects into three categories:

&#; Welds

&#; Edges

&#; Steel surfaces in general

When inspecting for defects, pay close attention to welded joints, cuts, punctures and scoring. Run a gloved hand over the distressed areas looking for places where it catches to identify protrusions. Some protrusions, such as weld spatter, can be abrasively blasted, others will require grinding.

When asked to grind a protrusion with power tools, it is crucial to avoid leaving any surfaces rough, with burrs or burnished. When grinding defects, it is also important to not reduce the mass of the substrate to less than that of the surrounding metal, also to not to grind in a way that creates excessive heat &#; both cases will weaken the steel. Also, ground defects should be inspected afterwards by the glove test to ensure that the process did not create sharp edges.

Knowing what kinds of defects to look for, let&#;s look at the different grades of surface prep.

 

Making the Grade

There are three grades for steel surfaces dealing with visible imperfections as set out by ISO -3:

P1 Light Preparation: no preparation or only minimum preparation needs to be carried out before application of paint;

P2 Thorough Preparation: most visible imperfections are remedied;

P3 Extensive Preparation: the surface is free from significant visible imperfections.

When deciding on the preparation grade, make sure you&#;re preparing the surface to specifications, especially considering that a single fabrication may require different prep grades &#; for instance: P1 on the inward-face and P3 for the outside wall.

Getting a consensus between all relevant parties on preparation grades and specific visible imperfections before you start can save time and effort.

 

Imperfections, Defects and Prep Grades Revealed

 

 

 

 

Weld Spatter

During the transfer of wire to weld, there is a disturbance in the molten weld pool. This disturbance, usually caused by the voltage being too low or amperage being too high, causes weld matter to fulminate onto the steel surface. 

Preparation Grades

P1 &#; Free from all loose weld spatter (a)
P2 &#; Free from all loose and lightly adhering weld spatter (a + b)
P3 &#; Surface must be free from all weld spatter. (a + b + c)

 

 

 

Weld Ripple/Profile

Surface oscillations in the weld pool solidifies as a ripple along the length of the bead profile. 

Preparation Grades

P1 &#; No Preparation
P2 &#; Surface shall be dressed to remove irregular and sharp-edged profiles
P3 &#; Surface shall be fully dressed, i.e. smooth

 

 

Weld Slag

The deoxidisation process between the flux coating, air and surface; the result of which is deposited as a residue on and around the weld bead. 

Preparation Grades

P1 &#; Surface shall be free from welding slag
P2 &#; Surface shall be free from welding slag
P3 &#; Surface shall be free from welding slag

 

 

Undercut

A weld flaw; a groove or crater near the toe of the weld bead resulting in a weak bond and is prone to cracking. 

Preparation Grades

P1 &#; No preparation
P2 &#; Surface shall be free from sharp or deep undercuts
P3 &#; Surface shall be free from sharp or deep undercuts

 

 

Weld Porosity

Weld porosity is a common welding defect. When you apply a torch to treated metal, gasses are released and absorbed into the molten metal. As the metal cools, the gasses are released from the metal, forming pores. 

Preparation Grades

1 &#; Visible

2 &#; Invisible (might open after abrasive blast cleaning)

P1 &#; No Preparation

P2 &#; Surface pores shall be sufficiently open to allow penetration of paint

P3 &#; Surface shall be free from visible pores

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Weld End Crater

Incorrect weld technique; this defect occurs where the arc has been broken, resulting in a crater. 

Preparation Grades

P1 &#; No preparation

P2 &#; End craters shall be free from sharp edges

P3 &#; Surface shall be free from visible end craters

 

 

Rolled edges

When an edge has formed to transition from one surface plane to another surface plane in a gradual curvilinear fashion. 

Preparation Grades

P1 &#; No Preparation

P2 &#; No Preparation

P3 &#; Edges shall be rounded with a radius of not less than 2 mm

 

 

Edges: punch, Shear, Saw, Drill

Edge transitions made during fabrication by punching, shearing, sawing or drilling tools. 

Preparation grades

P1 &#; No part of the edge shall be sharp; the edge shall be free from fins

P2 &#; No part of the edge shall be sharp; the edge shall be free from fins

P3 &#; Edges shall be rounded with a radius of not less than 2 mm

 

 

Thermally Cut Edges

When a plasma, oxygen fuel or other thermal process is used to cut steel. 

Preparation Grades

P1 &#; Surface shall be free from slag and loose scale

P2 &#; No part of the edge shall have an irregular profile

P3 &#; Cut face shall be removed and edges shall be round

 

 

Pits and Craters

When corrosion, of an extremely localised variety, leads to small perforations on the steel substrate.

Preparation grades

P1 &#; Pits and craters shall be sufficiently open to allow penetration of paint

P2 &#; Pits and craters shall be sufficiently open to allow penetration of paint

P3 &#; Surface shall be free from pits and craters

 

 

Shelling/slivers/hackles

A layer of corrosion in steel causing the corroded surface to separate and lift, leaving an interlocking flaky shell texture. 

Preparation grades

P1 &#; Surface shall be free from lifted material

P2 &#; Surface shall be free from visible shelling

P3 &#; Surface shall be free from visible shelling

 

 

Roll-overs/cut laminations

A fabrication defect cutting into the surface causing a thin protruding slice. 

Preparation grades

P1 &#; Surface shall be free from lifted material
P2 &#; Surface shall be free from visible roll-overs/cut laminations
P3 &#; Surface shall be free from visible roll-overs/cut laminations

 

 

Rolled-in extraneous matter

A defect occurring during the fabrication process where foreign matter is caught under a mechanical roller and embedded into the steel surface. 

Preparation Grades

P1 &#; Surface shall be free from rolled-in extraneous matter

P2 &#; Surface shall be free from rolled-in extraneous matter

P3 &#; Surface shall be free from rolled-in extraneous matter

 

 

Grooves And Gouges

A disfigured burrow or rough opening found on a steel surface, typically caused by mishandling. 

Preparation Grades

P1 &#; No preparation

P2 &#; The radius of grooves and gouges shall be no more than 2 mm

P3 &#; Surface shall be free from grooves and gouges 

 

 

Indentations and Roll Marks

A deep furrow or recess in the steel substrate, typically caused by mechanical manipulation.

Preparation Grades

P1 &#; No preparation

P2 &#; Indentations and roll marks shall be smooth

P3 &#; Surface shall be free from indentations and roll marks

Key Takeaway

Surface defects should be identified during the inspection, with a plan to remove them discussed before the job begins. However, if you encounter surface defects that look problematic during the course of blasting, report the defects to the project manager before taking action.

For more information, please visit Surface Inspection System For Steel.