Metallographic Sample Prep: Building a Grind and Polish Sequence That Holds Up

Metallographic sample preparation is a sequence, not a set of separate jobs, and at LabSupplies.com the question we get most from metallurgical and QA labs is where a stubborn scratch came from. The answer is almost always two stages upstream of where it was noticed. Sectioning decides how much damage grinding has to remove; grinding decides whether polishing has anything to do but chase scratches. Get the order and the overlap right and the final step becomes short and boring, which is what you want.

The Four Stages, and What Each One Owes the Next

Stage	Job	What it costs the next stage if done badly
Sectioning	Cut the sample with the least deformation	A burned, work-hardened face that grinding must remove before real structure appears
Mounting	Hold the sample square and support the edge	Edge rounding, and a sample that rocks under load
Grinding	Remove sectioning damage, flatten, step down grit	Retained coarse scratches that polishing cannot reach
Polishing	Remove the finest grinding scratches, no relief	Nothing — this is where the sequence either pays off or does not

Sectioning: Match the Wheel to the Material

Abrasive choice follows the material: aluminum oxide for ferrous metals and steels, silicon carbide for non-ferrous and most non-metallics. Use the wrong one and the wheel either glazes or wears far too fast, and both outcomes put heat into the cut face. Our sectioning wheels are listed by diameter and arbor so you can match your saw directly — the 10 inch aluminum oxide wheel with a 1.25 inch arbor (AM1067) and the 12 inch silicon carbide equivalent (AM1066) are typical, and both are stocked in packs of 10 because a production saw goes through them.

Two habits that pay for themselves: flood the cut properly, and keep a dressing stick (AM1725) at the saw. A glazed wheel cuts by rubbing, and rubbing is heat.

Grinding: Step Down, Rotate 90°, and Do Not Skip Grades

The working rule is that each step has to erase the previous step's scratch pattern. Rotating the sample 90° at every change makes that visible: while scratches at the old angle remain, the step is not finished. Time is not the test, because hardness and load change the rate.

A practical progression, all stocked in 12 inch plain back so one order covers the run: 240 grit (AMR0055), 320 grit (AMR0045), then 600 grit (AMR0025), with 1200 grit (AMR0008) where the method asks for a finer entry into polishing. For heavy stock removal after a rough cut, a 60 grit PSA-backed disc (AMR0195) or a 120 grit zirconia PSA cloth (AMR0386) does the planar work faster than stepping a fine paper to death.

Holding the sample matters more than it sounds. A 2 inch magnetic sample holder (AR5510) keeps load square across the face; a hand-held mount tilts, and a tilted mount rounds the edge you probably care about most. Grit sequence, PSA versus plain back and disc diameter get their own treatment in our silicon carbide grinding paper guide.

Polishing: Two Families, One Decision

Alumina handles most routine ferrous and softer non-ferrous work, and it is the cheaper consumable per sample. A 0.3 micron alumina suspension (AM0827) is a standard final step. Diamond earns its price where hard phases, carbides, ceramics and coatings would otherwise stand in relief while the matrix polishes away around them.

Whichever abrasive you run, the cloth is half the result. A napped 12 inch cloth (AM0539) carries suspension well for final work; a low-nap or hard-woven cloth holds flatness better on a sample with hard phases. The full alumina-versus-diamond decision, including when relief is the thing you are actually fighting, is covered in our alumina and diamond polishing guide.

Sizing the Order to Real Throughput

This is the part that gets decided by guesswork more often than by measurement. Grinding discs and polishing cloths are consumed continuously, which is why they are stocked in packs of 100 and 10 rather than singly. The way to size a standing order is to count discs consumed against samples completed over two or three weeks, then buy to that rate with a margin — not to buy the smallest pack repeatedly and discover the shortage mid-run.

Consumable	How it is stocked	How to size it
Sectioning wheels	Packs of 10, by diameter and arbor	To cuts per week, plus one pack of the second abrasive type
SiC grinding discs	Packs of 100, per grit and diameter	To measured discs-per-sample across the whole grit ladder, not one grade
Polishing cloths	Packs of 10 to 25	To polishing hours; nap collapses before the cloth tears
Suspensions and powders	6 oz bottles to gallons; 1 lb to 5 lb powder	Bottles while validating a method, bulk once the method is fixed

One caution on bulk: buy the large format once the method is settled, not while you are still changing it. A gallon of the suspension you have decided against is a more expensive mistake than a few small bottles.

The full range is in metallographic abrasives and polishing, with mounting consumables in metallographic mounting and sample prep. Most of the range carries OEM cross-references in the listing title, so if your method names a part number you can search that number directly.

Frequently Asked Questions

What is the correct grit sequence for metallographic grinding?

Start at the coarsest grit that removes the sectioning damage, then step down without skipping more than one grade at a time. A common planar-to-fine progression is 120 or 240, then 320, 400, 600, then 800 and 1200 where the method calls for it. Each step must remove the scratch pattern from the step before it, which is why skipping grades shows up as retained scratches in the final image rather than as a time saving.

How do I know when to move to the next grinding step?

Rotate the sample 90 degrees at each new step and grind until every scratch from the previous orientation is gone. When the surface shows one uniform scratch direction and no stragglers at the old angle, that step is finished. Time is not the criterion, because sample hardness and applied load change the rate.

How many samples does a pack of grinding discs actually prepare?

That depends on sample hardness, load and how aggressively you section, so the honest answer is that you measure it in your own lab. Count discs consumed against samples completed over a couple of weeks, then size your standing order to that rate. Grinding discs are sold in packs of 100 precisely because a working lab consumes them continuously rather than occasionally.

Do I need diamond polishing, or is alumina enough?

Alumina suspensions and powders finish most routine ferrous and softer non-ferrous work. Diamond earns its cost on hard phases, carbides, ceramics and coatings, where alumina removes the matrix faster than the hard particles and leaves relief. Many labs run diamond for the intermediate step and alumina for the final one.

Why does the cut-off wheel choice affect the polish?

Sectioning sets how much damage the grinding stages have to remove. A wheel that is too hard for the material burns and work-hardens the cut face, and that deformed layer has to be ground away before the real microstructure is exposed. Matching wheel abrasive and bond to the material shortens every stage that follows.

Setting up a new lab? Our LaunchLab program gives you 15% off everything for 12 months, metallographic consumables included. See if you qualify →

Sizing a standing order for your prep lab? Browse the full abrasives and polishing range. Browse the collection →

Send us your sample material, your saw and your grinder platen size at support@labsupplies.com and we will put together the grit ladder and pack sizes before you order.

Related Guides

• Silicon Carbide Grinding Paper: Grit Sequence and Backing
• Alumina vs Diamond Polishing
• How to Verify an OEM Cross-Reference Before You Order
• Crucible Selection for Carbon and Sulfur Analyzers
• Lab PPE Requirements Guide

Sources and Technical References

• ASTM E3 — Standard Guide for Preparation of Metallographic Specimens, ASTM International — store.astm.org
• ASTM E407 — Standard Practice for Microetching Metals and Alloys, ASTM International — store.astm.org

— By the LabSupplies.com Technical Team