What are stainless steel castings?

Stainless steel casting is a metalworking process used to produce custom stainless steel parts, in which molten metal is poured into an expendable ceramic mold for solidification, resulting in a solid stainless steel part of the desired shape.

Compared to other metalworking processes, stainless steel casting allows the manufacture of complex parts with fine details and good dimensional accuracy, thus reducing post-processing operations. It is also a good alternative to welded manufacturing processes because several stainless steel parts can be cast in one piece, which saves the time and cost of additional welding or assembly.

Casting stainless steel is the right way to go when both the properties of steel and corrosion resistance are required. The good corrosion resistance and high strength of stainless steel castings both make it popular in marine, food machinery, construction parts, medical and other applications. Therefore, stainless steel is the most popular material used in casting production.

Is stainless steel difficult to cast?

The answer is YES, comparing with other materials like cast iron or cast aluminum, stainless steel casting requires a higher melting point at around 2600˚F, while iron casting only needs about 2300˚F. That is to say, it is more difficult to melt stainless steel alloys, which take much more energy to melt. In order to cast, both stainless steel and iron could only be poured into molds at a much higher temperature than their melting points.

Regular casting materials melting point chart

Materials Melting point (°F) Melting point (°C)
SS 304, 304L, 304N 2550-2650 1400-1450
SS 316, 316L, 316N 2500-2550 1375-1400
SS 17-4 PH 2560-2625 1400-1440
Brass, Red 1832 1000
Brass, Yellow 1710 930
Copper 1983 1084
Iron, Wrought 2700 – 2900 1482 – 1593
Iron, Gray Cast 2060 – 2200 1127 – 1204
Iron, Ductile 2100 1149
Aluminum 1220 660
Aluminum Alloy 865 – 1240 463 – 671

What’s the best stainless steel grade for casting

Before deciding which grade of stainless steel is best for casting, let’s look at what are some of the more popular stainless steels used in casting. The following is a list of commonly used stainless steel grades with basic differences between them:

GradesFeaturesApplications
304Austenitic stainless steel with Ni content of more than 8%, food grade alloy, can be used to cast stainless steel components for both household and commercial applications. It is the most widely used stainless steel casting material.304 stainless steel castings can be used in environments where the air is less corrosive.Medical, food industry, chemical industry, mechanical equipment, pipe industry, automotive industry, etc.
316Also austenitic stainless steel with Ni content of more than 10%. For its higher Ni content, 316 stainless steel castings have better corrosion resistance than 304 stainless steel castings. Such stainless steel castings are better suited for the marine environment with relatively harsh air conditions or chemical materials that need to be contacted.Fire fighting, auto parts, marine hardware, chemical, pipeline, construction, decoration, food industry, etc.
304L / 316LThe mechanical properties are close to those of 304 and 316 materials. L represents lower carbon content, which makes the material softer, has good welding performance, and has more reliable corrosion resistance. The price is higher than that of materials of the same grade.Food, chemical, medical, plumbing, etc.
410 & 416Series 400 belongs to martensitic stainless steel, which is characterized by high strength, good processing performance and high heat treatment hardness, and does not contain Ni, so the corrosion resistance is weak.Auto parts, tools, knives, etc.
17-4 PH17-4 belongs to martensitic stainless steel with a Ni content of 3%-5% and good corrosion resistance. It has the highest strength in the stainless steel series and is usually used for products and components that are not prone to deformation.Military, medical, mechanical components, machine tools, turbine blades, etc.
2205Duplex stainless steel 2205 is a composite stainless steel consisting of 22% chromium, 2.5% molybdenum and 4.5% nickel-nitrogen.It has high strength, good impact toughness and good overall and local resistance to stress corrosion.Sporting, pump & valve industry, etc.

Analyzed in terms of composition, stainless steel is an alloy composed of a large amount of chromium content, usually no less than 10.5% by mass. Chromium is responsible for corrosion resistance, so any increase in chromium will make the metal more resistant to corrosion.

In addition to chromium, stainless steel is composed of other components such as molybdenum, nickel, titanium, copper, nitrogen, phosphorus, etc. Of all these minor components, molybdenum is absolutely unique. Why?

It greatly enhances the corrosion resistance of stainless steel. Furthermore, molybdenum distinguishes the two most common types of stainless steel which are stainless steel 304 and stainless steel 316. The difference between all the above stainless steel grades is their unique alloy composition. Due to the differences in chemical composition, so they have different physical properties.

That aside, it is prudent that you pick the most appropriate type of stainless steel before you begin the casting process. Remember that the advanced goal of your casting is to produce high-quality castings at a reasonable cost.

Therefore, the best(or most used) grades to use for casting are stainless steel 304 and stainless steel 316. Please note that both of these metals are in the 300-grade series of stainless steels, meaning they are chromium-nickel alloys.

Then which type is the most suitable 304 and 316? I would say that usually, both types are very useful. However, your choice between the two will depend on the circumstances in which you intend to use the metal. For procedures where your application requires good formability, then choose 304 over 316. The latter has less formability. Also, if you are considering working in an environment with many corrosives, such as marine and saltwater conditions, then choose 316. When we turn our attention to cost, 304 is the better choice because it is relatively inexpensive compared to 316. Finally, for applications that require greater strength and hardness, choose 316 over 304.

For more details about the materials’ differences, please refer to Casting  Stainless Steel Types and Grades.

The best method to cast stainless steel

There are many different popular casting methods for stainless steel, like centrifugal casting, sand casting, lost foam casting, investment casting, etc. But if you want me to name the most used one, I’d say investment casting.

In most foundries, stainless steel castings mainly operate in the investment casting process. In this method, molten metal is poured into an expendable ceramic mold. The mold is formed by using a wax pattern – a disposable piece in the shape of the desired part. The pattern is surrounded, or “invested”, into ceramic slurry that hardens into the mold. Investment casting is often referred to as “lost-wax casting” because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mold is destroyed during the process, parts with complex geometries and intricate details can be created.

Investment casting can make use of most metals, most commonly using stainless steel, aluminum alloys, bronze alloys, magnesium alloys, cast iron, and tool steel. This process is beneficial for casting metals with high melting temperatures that can not be molded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High-temperature applications are also common, which include parts for the automotive, aircraft, and military industries.

Stainless steel casting step-by-step process

1. Creating A Mold

Unless investment casting is being used to produce a very small volume (as is common for artistic work or original jewelry), a mold or die from which to manufacture the wax patterns is needed.

investment casting process-Produce Wax Pattern-s

2. Produce Wax Pattern

The wax is injected into the metal mold or die and allowed to solidify. The number of wax patterns is the same as the castings to be produced, each individual casting requires a new wax pattern.

investment casting process-assemble wax pattern tree

3. Assemble Wax Patterns Tree

Assemble the wax patterns to a gate or system. Multiple patterns are assembled to allow for large quantities to be produced at one time.

investment casting process-creating shell

4. Creating The Shell

Dip the pattern tree into ceramic slurry, the thickness of the ceramic shell depends of the size and weight of the part being cast. The average wall thickness is approximately 0.375 in(9.525 mm).

investment casting process-wax removal

5. Wax Removal

The hardened ceramic mold is turned upside down, placed in an oven, and heated until the wax melts and drains away. The result is a hollow ceramic shell.

6. Melt And Cast

The ceramic mold is preheated to around 1000 – 2000°F (550 – 1100°C) and filled with molten metal, liquid metal flows into the pouring cup, through the central gating system, and into each mold cavity on the tree.

investment casting process-casting removal

7. Shakeout And Casting Removal

Once the casting solidifies, break the ceramic shell, and cut the individual investment castings from the sprue. Finally remove the excess metal from casting body by grinding.

8. Finishing

If necessary, final post-processing sandblasting, grinding, and machining is performed to finish the casting dimensionally. To improve the mechanical properties of the parts, heat treating may be applied.

What are the advantages of stainless steel investment castings?

The greatest advantage of stainless steel is its excellent corrosion resistance. Unlike other materials, stainless steel investment castings have a built-in layer of chromium oxide that provides exceptional corrosion resistance. Due to its complexity and labor requirements, investment casting is a relatively expensive process – however, the benefits often outweigh the cost.

1. Large size range

Parts manufactured by investment casting are normally small, but the process can be used for parts weighing up to 250kg.

2. Versatile and intricate shapes

Great versatility, suitable for casting most metals. Very intricate castings can be produced easily. For example, where machine tools cannot reach.

3. Smooth surfaces and superior dimensional accuracy

Net-shape parts are easily achievable, and finished parts are often produced with no seam line so machining and finishing are reduced or eliminated. A 125 micro finish is standard, and even finer finishes are not uncommon.

4. High strength

The metal load of the investment casting process is 90-95%, a value similar to that of Injection Moulding.

5. High-cost efficiency in large volume

For high-volume production, the time and labor saved by eliminating or decreasing secondary machining easily make up for the cost of new tooling. Small casting runs are less likely to make up for the investment. Generally, investment casting is a logical choice for a run of 25 parts or more.

Possible defects of stainless steel casting

The melting point of cast steel is relatively high, at this condition the steel is easy to be oxidized. Furthermore, the liquidity of the steel is poor, so the shrinkage is big, its body shrinkage rate is 10 ~ 14%, and line shrinkage is 1.8 ~ 2.5%.

Due to the complexity of the casting process, there are steps that, when done incorrectly, can lead to casting defects, or undesired irregularities in the metal casting process. Such as cracks, gas holes, etc.

Some defects are tolerable, some are repairable, and some must be eliminated. In order to identify the nature of the defect, examine the possible causes and take appropriate corrective action, its location, appearance, shape and size must be determined.

Here we some major stainless steel investment casting defects:

stainless steel casting gas hole

Gas hole exists on the surface or internal area of the casting. Gas or porosity appears as a round, smooth-walled cavity, it may appear as a slightly oxidized surface of varying diameter, a bubble formed in the casting during cooling. This occurs because most liquid materials can hold a large amount of dissolved gas, which is expelled when the metal solidifies. Gas porosity may present as pores on the surface of the casting or porosity may be trapped inside the metal. Internal holes are usually pear-shaped. Choke holes are irregularly shaped and have a rough surface. Surface gas holes can be detected by visual inspection, however, subsurface holes can only be detected after machining.

stainless steel casting shrinkage cavity

Shrinkage defects occur when casting sections change at the feed gate to the casting, or when the feed metal flow is not available to compensate for shrinkage as the thick metal solidifies during cooling. Shrinkage defects usually present as rough holes in the casting surface or internal. The grains of the shrinkage cavity are big and thick. Cavities might also be paired with dendrite fractures or cracks. Large shrinkage cavities can undermine the integrity of the casting and may cause it to eventually break under stress.

stainless steel casting hot tear

Hot tear crack defects, also known as shrinkage cracks, occur when the molten metal is restricted from shrinking by the ceramic shell mold during cooling. The appearance of the crack is linear or irregular curve. The surface of the hot tear crack is dark grey or black due to strong oxidation, and no metallic luster. The fracture surface of the cold crack is clean, with a metallic luster. General casting crack can be seen directly. However, inside cracks can only be found by using other methods.

Cracks are often associated with defects such as shrinkage, and slag inclusion. They occur mainly in the inside of the pouring corner, in the thick joints, and in the hot section between the pouring riser and the casting.

Coldshuts occur when the two metal streams do not fuse properly in the mold cavity, resulting in discontinuities or weak spots within the casting and a cracked appearance. Potential causes are low pouring temperature, slow pouring speed, or the shell being too cold.

When the coldshut is serious, it will become “owe cast”. Coldshut often appears in the top wall of the casting, thin horizontal or vertical plane, or in the joint area of thin & thick wall thickness.

stainless steel casting inclusion

There are 2 main types of inclusion, slag inclusion and sand inclusion.

Slag inclusion is a defect where non-metallic materials create pockets or ribbon-like entrainments in the casting. A slag hole is a clear or indistinct hole in a casting. It is filled wholly or partially by slag and has an irregular shape. Small shapes of slag are not easily detected. After removing the slag, it will show smooth holes, usually distributed in the bottom area of the pouring position, around the runner cross-section, or near the corners.

Sand inclusion is very common. It is formed in the casting surface or internally, and is in a relatively regular shape.

What post-treatments can be used for stainless steel castings?

 

1. Solution heat treatment

To make the structure and composition of stainless steel castings uniform, solution treatment is the most common heat treatment method used for stainless steel casting parts. Besides, this method can also eliminate hardening for subsequent machining processing.

After the stainless steel is heated to about 1100°C, the carbide phase is completely or largely dissolved and the carbon is dissolved in the austenite, and then rapidly cooled to room temperature so that the carbon reaches a supersaturated state, a method known as solution heat treatment.

solution heat treatment

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2. Post machining

Although higher dimensional accuracy and surface finish can be achieved with stainless steel investment casting, this does not mean that all stainless steel castings products can skip machining. When there are tighter tolerances and surface finish requirements that the casting cannot meet, machining must be applied.

3. Surface finishes

Investment casting can achieve good dimensional tolerances, and excellent surface finish. Most machining can be saved. Sometimes, however, you might need a better surface finish. Therefore, the surface finish or surface finish of investment casting becomes an important post-treatment method to make the casting surface smooth.

stainless steel casting shot blasting surface

Shot Blasting


The most widely used surface treatment method is used to clean, strengthen (peen) or polish the surface of stainless steel castings. Low cost.

stainless steel casting mirror polishing

Mirror Polishing


The brightest surface polishing way can achieve a very smooth and shining surface like a mirror. Only suitable for outside areas of stainless steel castings.

stainless steel casting electropolishing

Electropolishing


Electropolishing is usually used to remove surface tiny burrs and improve the brightness and the corrosion resistance of stainless steel castings.

stainless steel casting pickling & passivation

Pickling & Passivation


Pickling is a chemical treatment way to eliminate surface dirt like oxide skin, rust, welding spots, etc. And passivation is a process that forms a new abundant chromium protective layer, thus improving the anti-oxidation ability of stainless steel castings.

stainless steel casting PVD Coating

PVD Coating


PVD coating is a surface finish way to color stainless steel castings. After finished castings, the casting parts are better to mirror polished or electropolished. There are various colors for stainless steel castings. Such as gold, rose gold, etc.

stainless steel casting Vibratory Polishing

Vibratory Polishing


Used to burnish, deburr, descale, clean, and brighten stainless steel castings. The surface looks smooth and a bit shining, but with less brightness than a mirror-polished surface. Compare to other deburring methods, Vibratory Polishing can ​prevent stainless steel castings from being bent or distorted.

Applications of stainless steel casting

Why Choose SIPX

With over 10 years of experience, Sipx is well-versed in this field. Yes, you are dealing with masters. You can trust us with your needs and expectations.

You will save 20-30% of the cost because of our excellent production capacity. Furthermore, we only insist on making high-quality casting products.

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