Huck vs. Welding

Welding compared to boltingBolting Welding

Flanges required Seam welds possible

Pre-coated parts Cleaned parts, paint afterwards

Pre-manufactured parts Often fabricated on site

Assembly Fabrication

Joint responsibility with design engineer

Joint responsibility with welder (easy to add that little bit more, quality)

Cost visible on Bill of Material Cost mainly in labour time

Normally more flexible joint Joint can be stiffer than material

Welding compared to bolting

F

F

� Seam joint for welding or lap joint for bolting

The Welded situation

� Preparation of joint

� Welding of joint

� Painting after assembly

– Often done by same skilled labour

� Cracking of weld

– repair

Joint failure� Cracking of end of joints

� Highest stress levels

� Weld impurities

� Hydrogen embrittlement)*

� Warping of material possible

� Fatigue or brittle failure

)* Source: The welding Journal, Feb 1990, Hydrogen-Induced Cracking along the Fusion Boundary of Dissimilar Metal Welds, M.D. Rowe et all

Cracking and hardness traverse near the fusion boundary

Joint failure

Dual Phase HAZ

Course Grained HAZ

Weld Material

Source: Oil & Gas - Microstructural change in high t emperature heat-affected zone of low carbon... D.Ca rrouge

Even it seems that the materials are joined together, there is still differences in grain structure, especially in the Heat Affected Zone, creating higher internal stresses. The HAZ is normally the point of failure of a weld.

Cost comparison: Welding vs Huck

Welding cost mainly labour costs

� Labour time:

– Grinding

– Cleaning

– Rigging up

– Cleaning, inspecting weld

– After-painting

� Labour rate

� Increase in material cost (+50%)– Pre-punched holes

– Fasteners

� 26% Saving compared to welding

72.3

16.1

11.3 0.4LabourMaterialsEnergyOther

)* Source: Welding-Related Expenditures, Investments, and Productivity Measurement in U.S. Manufacturing, Construction, and Mining Industries May 2002

Welding related production expenses)*

26

43

1

24

6SavingLabourMaterialsEnergyOther

Huck bolting related production expenses

(-20%)

(-30%)

Bolting compared to welding

� A more ductile and flexible joint is created.

� Stress limitations down to maximal contact loads between parts and maximal performance of fastener.

� In a ‘good’ designed joint, the forces travel through the joint and not the fastener, due to the high friction between the plates

Blind rivet versus welding

Bulbing blind fasteners:

• Wide bearing area

• Thin sheet applications

• No pull through during impact

• Controlled failure (of sheet material)

• High energy uptake

• Brittle failure of weld

Huck versus Welding

Pro’s:Easy visual inspectionNo brittle failure, the fastener will fail ductile (compared with welding)Joining of different materials possible, using the appropriate cathodic protectionNo Heat Affected Zone as with weldingJoint is vibration proof

Con’s:Perceived expensive toolingAddition of parts on Bill of MaterialWeakening of material due to necessity of holes to install fasteners inOverlap necessary, no overlap necessary with welding.

Maximum allowed stress levels

� Fatigue:

Tokyo Institute of Technology

Maximum allowed stress levels

� Bolted joints can have same or higher stress levels than welding

In the long term: convincing evidence

Lockbolt grade 8.8

BOM blind rivetBolt and retaining

nut grade 8.8

Summary

• Welds difficult to inspect• Expensive equipment needed• Or not inspected

• Specialized workers• Expensive• Training

• Increased human error• More chance of making

mistakes

• Time for complicated welds• Production time decrease• Possible interference with

multiple welders

• Secondary cleaning and polishing etc.• Higher production time.• More expense

• Tooling• Expensive• Safety issues

• And of course:

SAFETY…

Environment, Health and Safety

• Expense• Costly ventilation systems to

remove harmful gases:• 20-40% Iron Oxide• 2-8% Manganese• Arsenic• Lead• Chromium• Nickel

• Protection such as aprons, gloves and masks:

• Health Risks• Even with ventilation and

masks, the risks of illnesses are high:

• Lung cancer• Skin cancer• Retina damage• Asthma• Bronchitis• Renal Failure• Infertility• Possible hearing problems

Final note

� To change from a welded design to a bolted/riveted design, in many cases, a full redesign is required to suit lap joints and calculate the load cases.

� Changing over from welding to threaded fasteners might be an intermediate solution which is eventually easier to convert.