Grinding is a machining process that is used for material removal and finishing of stainless steel workpieces.
However, the heat-sensitive nature of stainless steel can pose several challenges in the grinding operation.
So, is grinding stainless steel possible? What are the things to consider when grinding stainless steel?
Grinding stainless steel can throw up various challenges due to its hardness, heat sensitivity, and chemically reactive nature that can easily contaminate the surface of the workpiece. Therefore, it is advised to carefully select an abrasive that minimizes heat generation and prevents contamination of the workpiece.
This article provides a detailed guide on grinding stainless steel by going through the challenges faced during the process and shedding some light on techniques to minimize those challenges.
What's in this article?
- Grinding stainless steel- Is it possible?
- Process control in grinding stainless steel
- How to select the right abrasive for grinding stainless steel?
- Challenges faced in grinding stainless steel
- Does grinding affect the anti-corrosive properties of stainless steel?
- Applications of grinding stainless steel
- Grinding vs finishing
- Final thoughts
- Frequently asked auestions (FAQ)
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Grinding Stainless Steel- Is it Possible?
A simple answer to that question is: Yes, grinding stainless steel is possible. However, it should be performed under optimal conditions by using the right tool and technique.
The properties that make stainless steel difficult to grind are its low thermal conductivity, nasty chemical reaction, and high ductility.
Due to the low thermal conductivity of stainless steel, the heat generated during the process is not readily dissipated, causing the temperature of the workpiece to rise during the grinding operation.
Apart from that, the high chromium content of stainless steel leads to the formation of chromium oxide which is harder than the aluminum oxide abrasive wheel, causing excessive wheel wear.
The high ductility of stainless steel promotes long chip formation during material removal, which often blocks the grinding zone, thus impacting the grinding operation.
With the proper selection of tools and by maintaining good process control, the grinding of stainless steel is possible.
Generally, stainless steels are available in different grades: austenitic stainless steel (304 and 316), ferrite grade (430), martensitic (410 and 440), precipitation-hardened stainless steel, and duplex steel.
Among these grades, austenitic grades have fair machinability and are suitable for grinding. The ferritic grade is formable but difficult to grind. Martensitic steel is easier to grind than ferritic and austenitic stainless steel.
The goal of stainless steel grinding is to keep it as cool as possible throughout the process. Excessive heat at the workpiece can cause dis-coloring and contamination.
Stainless steel has a very low grinding ratio (the amount of material removed per unit of wheel wear) when compared to other steel grades.
For reference, the grinding ratio of stainless steel varies from 6 to 12, while that of ordinary steel varies from 40 to 80.
Grinding of stainless steel is generally performed after machining, casting, or forging steel workpieces.
Process Control in Grinding Stainless Steel
|Grinding Pressure||Moderate and steady|
|Grinding Abrasive||Cubic Boron Nitrate (CBN)|
|Cutting Fluid Supply||Yes|
|Optimal Grit size||36-60 for roughing and above 60 for finishing|
As stainless steel is prone to contamination, it is advised not to grind it using grinding wheels that have previously been used on any other metal.
This eliminates the risk of chemical reactions, which can contaminate the workpiece surface.
As grinding stainless steel develops high frictional heat which can damage the workpiece and the cutting tool, it is advised to use a grinding fluid to minimize friction and enhance heat dissipation.
Moreover, grinding at the right speeds will reduce the grinding temperature and minimize the probability of weld reactions between wheel abrasives and workpiece chips.
Generally, it is advised to maintain a constant movement of the stainless steel workpiece during grinding to avoid heat accumulation in a local area, leading to discoloration.
Both insufficient pressure and wheel grazing can cause stainless steel to produce a darker spark than carbon steel during grinding.
Therefore, it is important to maintain steady pressure when grinding stainless steel.
In order to keep the wheel in good shape, only a light trim and occasional dressing are needed. This enhances the durability of the wheel and the performance of the grinding operation.
How to Select the Right Abrasive for Grinding Stainless Steel?
Right abrasive selection can simultaneously save expenses and boost productivity. You can find a variety of abrasive materials, each has its own unique trade-offs between cost, durability, and performance.
A grinding abrasive must be harder than the substance it is grinding. Therefore, it is crucial to use the proper abrasive when grinding stainless steel.
Apart from that, stainless steel is a non-magnetic metal and therefore does not adhere to the grinding wheel with ferrous abrasive.
However, its ductile nature can lead to clogging of the abrasive grit, leading to poor grinding efficiency.
Abrasive products with the term "Stainless" or "inox" on their labels are specifically designed for grinding stainless steel.
Based on the Abrasive type
The heat-reducing abrasives specifically formulated for use with stainless steel are discussed here:
Cubic Boron Nitride (CBN)
The most suitable grinding wheels for stainless steel workpieces are Cubic Boron Nitride (CBN) wheels.
Due to high hardness, grains chips off easily, and a negligible tendency to attract iron group elements helps CBN grinding wheels to not clog readily during grinding. Thus, offering a smoother operation.
CBN grinding wheels require comparatively less grinding force to perform the operation. This minimizes heat generation during the process, making it suitable for grinding stainless steel workpieces.
However, CBN grinding wheels suitable for grinding stainless steel are comparatively much more expensive than the ordinary grinding wheels.
When looking for a CBN grinding wheel for grinding stainless steel, it is recommended to have an abrasive with grain size F120, bonded together with a resin, and has a concentration of around 100%.
Monocrystalline Fused Alumina
Monocrystalline fused alumina (MFA) has a unique microstructure and high hardness that creates numerous durable cutting edges.
It has good grinding performance (next to CBN). Its grains have a spherical arrangement that avoids cracking and the building of residual stress during the grinding of stainless steel.
When grinding austenitic grade stainless steel, MFA with a green silicon carbide grinding wheel is used to greatly reduce the required cutting force and eliminate the risk of overheating.
Lastly, the MFA wheel is more environmentally friendly, due to less sulfur involvement in its production.
Zirconia Aluminum Oxide
Zirconia grain outperforms aluminum oxide grain in terms of sharpness, cutting speed, and durability.
Additionally, zirconia has better heat dissipation, which is beneficial for slicing through thick stainless steel plates.
Use T-grade bond for fast cutting and longer life.
Stainless steel and other steels are ground effectively and efficiently by zirconia T27 grinding wheels.
Based on the Abrasive Size
The abrasive grain size has a direct impact on surface roughness. Grinding stainless steel can block the grit of the abrasive, rendering the tool useless.
This issue is prominent when using a fine-grit abrasive for grinding stainless steel.
It is demonstrated that grain sizes 36-60 are for rough grinding and 60 and higher are for finish grinding of stainless steel.
Based on the Bond Type
The abrasive used with the grinding wheel must be durable with the ability to endure high impact loads. These properties are decided by the bond type of the grinding wheel.
Grinding of stainless steel can be performed with a vitrified bond grinding wheel. It is waterproof, highly heat resistant, corrosion resistant, and has a high performance.
Its downside is that it is brittle and cannot endure significant impact or bending.
On the other hand, grinding wheels with resin bonds are used for cutting and centerless grinding of stainless steel. It has great strength and elasticity, is shock-free, and is suitable for higher speeds.
However, grinding stainless steel with resin bonded wheel requires constant coolant flow as the resin bond softens if the temperature exceeds 150°C.
Challenges in Grinding Stainless Steel
Stainless steel has been used for a long time in many applications where strength, durability, and corrosion resistance are important.
Unfortunately, from a metalworking standpoint, the same characteristics that make it ideal for heavy industries also make it difficult to work with.
Overheating of Workpiece
Stainless steel undergoes considerable plastic deformation and temperature increase during the grinding process. On occasion, the cutting pressure of stainless steel might reach 90GPa.
The greater force exerted by the grinding wheel raises the grinding zone's overall temperature, raising the workpiece's local temperature up to 1000–1500°C due to the stainless steel's low heat conductivity.
Because of this, the surface suffers burns easily and causes cracks in the workpiece.
In general, machineable and grindable alloys are easier to grind and break up into small chips.
Such alloys clean themselves and keep the grinding zone clean, but stainless steel forms longer chips due to its high ductility.
Increased Cost of Grinding Process
Higher grinding forces associated with stainless steel grinding blunt the abrasive grains and deteriorate the grinding wheel.
Looking at the costs associated with grinding, the rework on stainless steel costs almost double when compared to carbon steel or free-machining steel.
Since, the application demands a contaminant-free environment, abrasives used with stainless steel may be more costly.
Stainless steel consists of nickel as one of its constituents and grinding stainless steel can produce fine dust of stainless steel particles suspended in the air.
This dust consists of nickel, which when inhaled by the operator can damage the lungs.
Therefore, it is advised always to wear a respirator to minimize health hazards.
Does Grinding Affect the Anti-corrosive Properties of Stainless Steel?
Yes, grinding does affect the anti-corrosive property and renders stainless steel prone to rusting.
Chromium and nickel add corrosion resistance properties to stainless steel, and grinding removes the protective coating of chromium oxide leaving the surface prone to corrosion.
However, the chromium content present in the workpiece reacts with the ambient air and forms a "passive" layer of chromium oxide to protect the surface from corrosion.
Therefore, the chromium oxide layer keeps regenerating until there is enough chromium content present in the stainless steel alloy to react with the ambient air.
However, there are various factors that can affect the protective chromium layer during the grinding process.
Overheating and Burning
High-speed grinding wheels, dull abrasives, and pressure can result in overheating of stainless-steel workpieces, and as the temperature reaches the 750-1550°F range, a common form of rusting starts to occur.
To avoid burning, supersized zirconia or ceramic abrasive products are preferred.
Excessive Pressure During Finishing Operation
When polishing stainless steel, do not bear down or apply undue pressure. This may simply cause further discoloration or heat tinting.
To avoid a build-up of heat in one region, keep the abrasive wheel moving throughout the surface.
During the finishing and polishing phases, avoid overworking the surface by employing too much pressure and abrasive speed.
An orange peel effect can occur when there is too much stress on the stainless steel surface during shaping or finishing.
This effect can be minimized by using fine grit abrasives, followed by repolishing the workpiece at a slower speed and with lighter pressure.
Cross-contamination With Steel
Grinding of stainless steel on a wheel that has been used for grinding other steel grades, such as mild steel, can result in contamination of the surface of the stainless steel workpiece.
This reduces the corrosion resistance of the workpiece.
Applications of Grinding Stainless Steel
Grinding stainless steel is often required for pre-weld, post-weld, and finishing operations in a variety of industrial and commercial applications.
It is performed for cleanup and removal of buildup dross after plasma cutting of stainless steel sheets. This is a pre-weld operation adopted for surface preparation.
Grinding weld connections for railings are done in order to provide visually appealing weld connections. To grind the weld, 40, 60, or 80-grain belt grinders are often employed.
If you want the best finish possible, you should look for tools that have speed regulators and utilize them correctly. This will help you obtain the best possible finish.
In commercial food service grills, surface weld bead reduction is a key challenge. Starting with rough grinding using 32-80 size grit discs followed by 40-220 grit size abrasive belts provides a smooth surface.
Stainless steel tubes often required a mirror finish, which can be performed using zirconia belt grinding of 80-120 grit size abrasives.
Following this, the surface is conditioned using a buffing wheel and polishing paste.
A satin-finish stainless steel for a pressure vessel is achieved by using a 40 to 320-grain belt grind followed by a finishing operation using a flap brush.
Grinding vs Finishing: Which One to Perform?
|Quickly removes burrs and extra weld material||Performed to achieve finish, usually done after grinding operation|
|Done with large grit abrasive wheel on a simple angle grinder or belt grinder||Done stepwise, starting from large grit abrasive to finer grit. Additionally may require felt cloth or polishing paste.|
|Leaves deep scratches on stainless steel surface||A scratch pattern, polished or mirror finish is achieved.|
The decision over which finish type to choose for the stainless steel workpiece, depends entirely on the client's specifications or what the final product will be put to use.
For example, a smooth finish might be used for aesthetics purposes or pressure vessels, while an elevator wall panel or railing will only require grinding to smoothen the sharp edges.
Stainless steel grinding might be more expensive when compared to grinding aluminum and carbon steel, as it requires special abrasives with good process control.
A wrong selection of abrasive type, tool, and operating method can damage the material and end up spending a lot more time and money on rework, scrap, and lost efficiency in production.
However, grinding of stainless steel is only recommended for high-value parts, where a high surface finish is of utmost importance.
For example, a painted layer of steel can work well for dry and indoor places fabrication purposes.
However, if you want superior corrosion protection without coating and also want to add some aesthetic value to the product, appropriately finished stainless steel will never disappoint you.
Frequently Asked Questions (FAQ)
Does grinding stainless steel produce toxic dust?
Yes, grinding stainless steel produces hazardous dust containing heavy metals like chromium and nickel, which can cause respiratory irritation if subjected to it for a prolonged period of time.
What cutting fluids are commonly used for grinding stainless steel?
Translucent emulation (EM2), Chemical type soluble fluid (EM4), Compounded mineral oil (MO2), and compressed air can be used as effective cutting fluids for grinding stainless steel.
What are the most common grinding defects in stainless steel?
Some most common stainless steel grinding defects include heat tints, embedded iron particles, sulfide inclusions, and grind marks.
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