For slewing bearing material, 50Mn or 42CrMo，how to choose ?

50Mn or 42CrMo? When selecting materials for slewing bearings, don't just look at the grade—consider the load capacity and operating conditions.

Having worked in slewing bearing supply at SWBTEC for so many years, I've noticed a rather interesting phenomenon: many original equipment manufacturers (OEMs) adopt a one-size-fits-all approach when selecting materials—either using 42CrMo across the board, believing that more expensive must be better, or using 50Mn throughout to save every penny they can. In reality, both of these approaches are flawed. When it comes to materials, suitability is more important than cost.

We deal with 50Mn and 42CrMo on a daily basis, so we know their respective characteristics inside out.

Let's start with 50Mn; it's the 'budget-friendly' option for small to medium-tonnage applications:

50Mn has a carbon content of between 0.48% and 0.56%, and a manganese content of 0.7% to 1.0%. It has average hardenability; after quenching and tempering, the surface hardness can reach around HB 200–240, and with induction hardening of the tooth faces, it can achieve HRC 50–55. For equipment such as small excavators under 10 ton, small cranes between 6 and 8 ton, and agricultural machinery, this is entirely sufficient.

The characteristics of these types of machines are: low operational intensity, low rotation frequency, and extremely tight cost constraints. The factory price of a small excavator is set at a certain level; if you fit it with a 42CrMo slewing rings, the price rises significantly, and customers simply won't accept it. We have used 50Mn for components in 6-ton compact excavators; after four or five years of service, the wear on the raceways remained within normal limits when inspected upon removal. Therefore, in scenarios involving small tonnages, light-duty applications and cost-sensitive requirements, 50Mn is our first choice.

As for 42CrMo, it is the 'cornerstone'  for heavy-duty applications and high-tonnage machinery:

The alloy composition of 42CrMo determines its significantly higher upper limits: the addition of chromium and molybdenum substantially improves hardenability; after quenching and tempering, it offers excellent core toughness; surface hardening can easily achieve HRC 55–60, and the hardened layer depth is more uniform. We have conducted comparative tests in the laboratory—under the same contact stress, the contact fatigue life of 42CrMo is nearly 30%-40% higher than that of 50Mn .

Therefore, for excavators over 20 ton, mobile cranes over 25 ton, and heavy-duty equipment such as grab buckets, we strongly recommend 42CrMo. I recall a client two years ago who was manufacturing wind turbine maintenance hoists; whilst the load was not particularly heavy, there were frequent starts and stops with significant impact loads. Initially, they wanted to use 50Mn to save costs, but we did not agree, and 42CrMo was ultimately used. The equipment has now been in operation for over two years with zero faults. Conversely, if 50Mn is forced into use for heavy-duty equipment, no issues may be apparent in the short term, but after a year or so of operation, indentations and spalling are likely to appear on the raceways; in severe cases, this can lead to excessive rotational clearance, leaving the original equipment manufacturer's after-sales service unable to keep up.

How should one choose for mid-range tonnages? The 15–20 ton range is the most challenging.

We have a rough internal guideline: for under 12 ton, 50Mn is safe to use; above 25 ton, 42CrMo is mandatory; between 12 and 25 ton, it depends on the specific operating conditions.

For example, with a 15-tonne excavator, if the work primarily involves earthmoving with moderate operating frequency, and the user is price-sensitive, using 50Mn with increased cross-sectional dimensions is a viable option—we have conducted finite element analysis, and by appropriately increasing the raceway diameter and ball size, the load-bearing capacity of 50Mn can cover most usage scenarios. However, for a 15-tonne machine operating in high-impact conditions such as mining or when fitted with a hydraulic breaker, do not hesitate to choose 42CrMo. Spending a few hundred yuan more on materials will ensure reliability without issues for two years.

Another often-overlooked factor is regional climate and operating habits. In the cold winters of northern, low-temperature impact toughness is crucial, and 42CrMo has a clear advantage; in the milder southern regions, the disadvantages of 50Mn are not as pronounced. Furthermore, some operators have poor habits, frequently forcing the slewing mechanism to perform heavy work under restricted rotation; in such cases, the material’s resistance to plastic deformation becomes a key factor, and 42CrMo proves more durable.

Our approach at SWBTEC

Our slewing rings manuals no longer simply state “Material: 50Mn or 42CrMo”; instead, we have divided them into three selection tiers:

•   Economy: 50Mn, suitable for general-purpose equipment under 10 ton

•   Standard: 50Mn with thickened raceways or 42CrMo with thin walls, suitable for standard operating conditions between 12 and 20 ton

•   Heavy-duty: 42CrMo, suitable for loads over 20 ton and high-impact applications

Ultimately, material of slewing bearing selection is not a black-and-white issue. When communicating with original equipment manufacturers (OEMs), we often use an analogy: 50Mn is like standard tool steel – it's good enough for the job; 42CrMo is like special alloy steel – use it when it's needed. Helping customers save money where they shouldn't is misleading; insisting they spend money unnecessarily is irresponsible. Once you've been in slewing rings industry long enough, you will realise that reasonable material standards are the greatest mark of respect for both OEMs and end-users.

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