Process Overview: Nut Projection Welding on High Strength Hot Formed Steel

1. Introduction

High strength hot formed steel offers exceptional mechanical safety due to its ultra high yield strength. That is why it is being adopted more and more widely in the automotive industry. Key applications include front and rear bumper beams, A pillars and B pillars, center tunnels, and front/rear door impact beams. Almost all of these components require nuts or bolts to be projection welded onto them.

Nut projection welders are the natural choice for body in white parts, thanks to their short welding time, minimal heat affected zone, and high efficiency. And for welding nuts and bolts onto high strength hot formed steel, resistance welding equipment remains the preferred solution.

2. Case Study

Example: A pillar nut projection welding for a passenger vehicle model

Base material: BTR165H hot formed steel, 1.8 mm thickness

Fastener: M10 projection flange nut with three segment crescent projections

Requirements:

Torque ≥ 130 N·m

Push off force ≥ 8 kN

No thread damage

No visible surface damage

2.1 Welding Machine

The machine used was an Agera DR-30000J capacitive discharge welder, with a ridge electrode applying vertical force.

2.2 Adjustment of Welding Parameters

2.2.1 Basic principles
In conventional resistance welding, three fundamental parameters are adjusted together: welding time, welding current, and welding force. Capacitive discharge welders, however, do not have a separate welding time adjustment. Welding current is reflected through the charging voltage. Therefore, parameter adjustment on a CD spot welder mainly involves matching the charging voltage and welding force. In special cases, a tempering voltage can be added to refine the grain structure.

2.2.2 Key challenges for this high strength hot formed steel nut welding application

Welding force setting – Hot formed steel is extremely strong, so higher force is needed to form a proper nugget. But the nut itself is relatively soft, and excessive force will crush the projections prematurely. Ideally, a saddle shaped force curve works best. In practice, a two step pressure profile is more common — it prevents the projections from collapsing too early while avoiding excessive spatter during nugget growth.

Charging voltage setting – Too high a voltage generates excessive heat, causing the nut metal to extrude or spatter. This leads to two problems: first, spatter reduces the effective fusion area (overheating), so push off force drops; second, the thread gauge may not pass. Too low a voltage results in insufficient penetration, causing weak push off force or cold welds.

2.2.3 Tuning method
When adjusting the two main parameters (charging voltage and welding force), always keep one fixed while adjusting the other — never change both at the same time. Experience matters here. If you have similar nut welding parameters from other applications, use them as a starting point. It will save time. Just remember that hotformed steel requires higher current and force. Start with the values you would use on ordinary material of the same fastener size, then fine tune through test welds and laboratory verification to find the optimum.

One more note: for production runs, increase the test derived values by 3- 5% to provide a margin against normal process fluctuations.

2.3 Confirmed Welding Parameters

2.4 Push off Force and Destructive Testing

Test equipment used:

Torque wrench

Universal testing machine

Thread plug gauge

Test results:

Torque > 180 N·m

Push off force > 12 kN

Threads: no abnormality

Surface: no visible damage

3. Additional Notes on Electrode Tooling

3.1 Electrode material
Hot formed steel projection welding requires extremely high welding force and very high peak current. This places strict demands on electrode material. The ideal electrode needs high softening temperature, good electrical conductivity, and sufficient hardness. Dispersion strengthened copper (alumina copper) offers all three — with a softening temperature around 950°C — making it the most suitable electrode material for this application today.

3.2 Locating pins
Common locating pin materials include bakelite, ceramic, and KCF. In recent years, some European and American automakers have started using silicon nitride for locating pins in hot formed steel projection welding. Its advantages are clear: longer service life (up to 200,000 cycles), excellent insulation, and high hardness. It is becoming a new favourite, though the high cost means it is not yet widely adopted. Still, we believe it points the direction for future developments.

3.3 Blow off to reduce metal extrusion
Due to the physical properties of hot formed steel and the “hard” welding schedule used, metal extrusion is common. Using compressed air to blow across the welding area during the weld cycle helps significantly. The airflow oxidises and burns off the extruded metal quickly, greatly reducing adhesion and keeping the threads clean and within spec.

4. Conclusion

Through testing on high strength hot formed steel projection welding, we have confirmed that — using the Agera capacitive discharge welder and a properly tuned process — all requirements can be met comfortably. In fact, the test results consistently exceeded the specified targets by a wide margin.

There is little doubt that capacitive discharge welding is one of the ideal solutions for high strength hot formed steel nut projection welding. It represents a significant advancement in welding technology, and brings real, practical capability to an industry that has been searching for reliable ways to join fasteners to today’s strongest automotive materials.