Mechanical Face Seal Cast Iron or Hardened Steel Rings: Review a Swap Before You Approve It

A quote arrives for a floating-seal group you normally buy with cast-iron rings. This time the rings are bearing steel, and the supplier says the size is the same. Treat that as a changed specification, not a like-for-like substitution. A material name or a hardness figure does not qualify a ring pair. The option has to be tied to the exact material code of both halves, the lubricant, the housing and the operating envelope the maker rates, and a changed pairing needs approval for your application.

What the April 2026 catalog establishes

The review needs a primary source, so this article reads one maker's catalog: Trelleborg's Mechanical Face Seals catalog, Edition April 2026. It describes Trelleborg's own products. It is not a universal ring-pair specification, and it says nothing about any KTSU part.

The catalog notes that these seals are also called lifetime seals, floating seals and heavy-duty seals. It describes two types, DO with an O-ring as the secondary element and DF with a diamond-section elastomer, and says both consist of two identical metal rings sealing against each other on a lapped face. It adds that seals can also be divided by their seal materials, so material is one of the ways the product line is split. Lubrication is stated to be necessary at all times, forming a thin film between the sliding faces.

For the metal parts, the catalog gives hardness and chemical composition for bearing steel (100Cr6), a chromium-based alloy and a nickel-based alloy. It then tells the reader to contact the maker's Customer Solution Center to identify a suitable material for their application. That is the catalog's own answer to the question "which one?": ask the maker, with the application in hand.

Where the catalog stops

Three points in the catalog matter for a bearing-steel-for-cast-iron swap.

First, cast iron is offered, but it is not described the way the alloys are. The part-number tables carry separate bearing steel and cast iron columns, and a note says that all sizes in bearing steel can also be requested in cast iron. The speed guidance also gives separate figures for cast iron and for 100Cr6, and they differ. In the Materials section, however, the hardness and composition tables cover the three alloys and not cast iron, and the text introduces "two different alloy compounds" before listing three. Because of that, the catalog alone does not let you compare a cast-iron ring with a steel ring on composition or hardness. The material code and its source for each ring has to come from the drawing or certificate for the exact reference.

Second, the catalog treats a change as a reason to consult. It says to get in touch when designing a new application, or when an existing one is modified because of changes in peripheral speed, temperature, pressure, lubrication or dimension. It also says the seals should not see maximum pressure and maximum speed at once. The speed and pressure figures are Trelleborg's for Trelleborg seals, so this article does not repeat them, and they are not limits for any other maker's group.

Third, the catalog disclaims its own reach. It states that its information is for general reference only and not for specific applications. It adds that application limits are laboratory maxima that may not be achievable in operation, and that customers must satisfy themselves of suitability. The lubricant also matters: the catalog says oil gives the best results for wear and service life, and that grease is possible but needs special attention to speed. The elastomer is a separate choice again, with its own compound table and temperature ranges.

Identity is not approval

Two questions get merged in a quote. Identity asks what both rings are made of and where that is recorded. Approval asks whether the maker has accepted this exact pairing for your lubricant, housing and duty. A certificate with a material code answers the first and not the second.

A hypothetical case shows the gap. Your current group is specified with cast-iron rings, and the quote offers bearing steel at the same dimensions. If the maker's catalog lists both materials for that size, the option exists, but that is all it shows. It does not show that the steel version is approved for your grease or oil, your speed or your housing. A higher hardness figure on one ring does not change that, because the rings work as a pair and the harder option is not shown to last longer in your conditions.

A mixed group, with one half in each material, is yet another configuration. This article does not endorse mixing unmatched halves, and such a pairing would need its own approval.

Record the change

Fill in a record for the quote. It is a recommended review aid, not a form required by any maker or standard.

Recording the catalog basis shows only that the option is published. It does not make the catalog's figures your limits. Every row left at "verify" or "unresolved" goes into the request to the supplier.

What to send, and what it settles

Write the request from the record: the exact references for both groups, the material code and its source for each ring, the elastomer, the lubricant, the operating conditions, and a request for written application approval or the maker's stated envelope for that reference. If the supplier answers with a label or a hardness figure, the record still reads unresolved.

For a seal or roller from KTSU, send the same details when asking what identity documentation is available. The public excavator undercarriage parts collection identifies the product family. It does not establish a ring material, an approval or an operating limit for any seal.

The review is complete when the record shows the material codes of both groups, the catalog basis with its edition, and the application approval as either received or still open.