Made-to-drawing 39NiCrMo3 stud with yellow zinc plating for Power Generation

The M16 × 110 mm made-to-drawing stud in 39NiCrMo3, supplied by EFS for the Power Generation sector, has two threaded ends measuring 34 and 40 mm respectively. As shown, one end is rounded and the component, manufactured from quenched and tempered steel, is finished with yellow iridescent zinc plating.

These features are best understood together and make this fastening system far from standard. In a made-to-drawing component, thread length, end geometry, material and surface treatment all respond to different assembly requirements.

In the energy sector, fastening systems may operate under high loads, vibration and repeated stresses. Under these conditions, material strength alone is not enough: geometry, thread engagement length and the correct configuration of the ends must also match the assembly.

In this case, quenched and tempered 39NiCrMo3 provides a combination of strength and toughness suitable for components subjected to dynamic loads. The made-to-drawing geometry allows the stud to suit the specific installation point, while the zinc coating provides surface protection defined according to the application requirements and the environment in which the stud operates.

Why can a stud have threaded ends of different lengths?

A double-ended stud does not necessarily need two symmetrical threaded sections. In some configurations, one end is screwed into a threaded hole while the other is used for tightening with a nut. The two ends therefore perform different functions.

The screw-in depth must provide the thread engagement required by the design. On the opposite side, the thickness of the components being clamped, any washer, nut height and the required thread projection must all be taken into account.

On our made-to-drawing 39NiCrMo3 stud, the flat end is screwed into the threaded hole, while the rounded end remains on the outside of the assembly.

For this reason, the 34 and 40 mm threaded lengths of the M16 × 110 stud are not simply a dimensional variation. They reflect the geometry of the assembly in which the component is installed.

What is the purpose of the rounded end of a stud?

On this component, the rounded end is the outer end, while the flat end is screwed into the threaded hole. The rounded profile removes the sharp edge from the exposed side and can make it easier to fit the nut.

The rounded shape also helps distinguish the outer end from the end intended to be screwed into the component. On a made-to-drawing stud, end geometry is therefore not merely a visual detail, but part of the assembly logic.

Quenched and tempered 39NiCrMo3 for mechanically loaded components

39NiCrMo3, material number 1.6510, is a nickel-chromium-molybdenum alloy steel traditionally used for mechanical components subjected to loads, vibration, torsion and fatigue.

Some may remember the former Italian designation NCM2, while cross-reference tables also list designations such as AISI/SAE 9840 and 40NCD3. These are approximate correspondences and should not be treated as automatic equivalents.

The combination of nickel, chromium and molybdenum promotes hardenability and toughness. After quenching and tempering, the material can achieve a balance between mechanical strength and the ability to withstand dynamic loading.

Yellow iridescent zinc plating: colour alone does not define the coating

The yellow-iridescent surface normally results from an electroplated zinc coating followed by a conversion/passivation treatment. The zinc provides the main corrosion protection for the steel substrate, while the subsequent treatment contributes to the surface characteristics and corrosion resistance.

However, “yellow zinc plating” is not a complete technical specification.

The yellow-iridescent colour alone does not define the performance of the coating. Modern passivation systems used in Europe generally rely on trivalent chromium, which has replaced hexavalent chromium, now heavily restricted under REACH and, in specific sectors, RoHS requirements.

To define a zinc coating for fasteners correctly, the specification should therefore include zinc coating thickness, passivation type, any sealant and the required corrosion performance.

Does yellow zinc plating indicate how corrosion-resistant the stud will be?

No. Colour alone does not determine performance. Corrosion resistance depends on the coating system, zinc thickness, passivation, any sealant and the test requirements specified for the application.

Where is yellow zinc plating used?

Yellow zinc plating is mainly used on steel components intended for industrial environments where moderate corrosion protection is required, without the need for coatings designed for severe exposure conditions.

In the Power Generation sector, this type of treatment may therefore be used on components installed in protected areas of the plant or within mechanical assemblies, where the specification requires corrosion protection compatible with an electroplated coating.

Hydrogen embrittlement

For a quenched and tempered made-to-drawing 39NiCrMo3 stud with an electroplated zinc coating, attention must also be paid to the possible risk of hydrogen embrittlement.

When does this risk need to be managed?

Hydrogen embrittlement is a real concern for high-strength components, typically from property class 10.9 upwards, when electroplated zinc coatings are used. In these cases, a post-plating hydrogen embrittlement relief heat treatment is normally specified in accordance with the applicable requirements and the coating process.

When geometry, material and coating must be managed as one specification

For this type of supply, EFS acts as the main contractor for special fasteners. Starting from the customer’s drawing and requirements, EFS selects manufacturers and processes that meet the specification and coordinates machining, heat treatment, threading, zinc plating and final inspections.

A special stud may differ because it features:

• a shank diameter corresponding to the thread pitch diameter or nominal diameter
• symmetrical or asymmetrical threaded lengths at the two ends
• identical or different thread tolerances
• a right-hand thread at one end and a left-hand thread at the other
• different thread pitches, for example M16 × 1.5 or M16 × 2.0

These are all features that make a made-to-drawing stud a non-standard component.

Controlling the entire manufacturing sequence ensures that the finished component complies with the drawing and the required documentation.

See also our made-to-drawing 39NiCrMo3 studs for machine tool applications.

Reference standards

ISO 4042:2022 – Fasteners — Electroplated coating systems