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80
Hardening and tempering
of fasteners
Dipl.-Ing. Peter Kreimer, Aichelin Ges.m.b.H, Fabriksgasse 3, A – 2340 Mödling
Today’s high quality requirements together with economic constraints have led the development of fasteners’ furnace
technology to continuous plants for mass-production, with belt-type furnaces resulting as the dominant design. This article
describes in general the process of hardening and tempering and the plant technology commonly employed in modern
fastener industry. Emphasis is put on precise process monitoring of the mass bulk flow with regard to the uniform final
properties like tensile strength and toughness of the single fastener.
Hardening and tempering is arguably the - Operating: media consumption. various grades in EN ISO 898-1 are often
most important heat treatment in the fas- - Service life: preventive maintenance. further reduced to 50 MPA, especially by the
tener industry. For automotive safety parts • Minimum environmental impact. automotive industry. Exactly observing the
this important production step is almost To achieve these objectives fasteners are theoretic temperature curves of hardening
always employed. Basis for the heat treat- nowadays chiefly heat-treated in continuous and tempering becomes secondary to the
ment of fasteners is the standard EN ISO 898- furnace lines tailored to customers’ needs homogenous distribution of the properties of
1 that prescribes a homogenous hardened and possibly geared towards an almost the single fastener and the continuous
and tempered steel structure. Carbon identical temperature – ambient gas/fluid monitoring of parameters like temperature,
contents of 0.25% to 0.35% result in best impact on the single pieces in the bulk flow. recirculation flow and protective gas
values for combined tensile strength and The characteristic statistic values composition is crucial to production.
toughness both for non- and low-alloyed
steels. A tensile strength ranging from 800 to
1,300N/mm² may also be achieved under
certain conditions and with limited flexibility
without final heat treatment (e.g. with cold
forging and ageing or employing micro
alloys) but these have had limited
application up to now.
Furnace technology
Industries’ high requirements for fastener
characteristics demand a specially designed
furnace, reliable plant technology, field
experience and knowledge of physical and
metallurgical process parameters and their
influence on heat treatment results. This is
especially true for mass-produced high-grade
fasteners with diameters ranging from
smaller than 2 to bigger than 50mm.
Generally it is expected that every single
piece, without having any defect at all, will
Fig 1 Partial view on a hardening and tempering line with
have defined properties with small a nominal throughput of 2,000kg/h
dispersions of hardness, tensile strength,
toughness and fatigue strength among describing the degree to which the required Batch type furnaces due to their economic
others. properties are satisfied are Cm(k) and Cp(k) disadvantages with large production runs are
for machine and process respectively, with used for special applications like positioning
The furnace plant design targets may be the subscript k further indicating the position with long and slim fasteners that are subject
summed up as: of the scatter band between the specified to bending or for flexibility demands with
• Highest process capability within narrow tolerance limits. The k values can most times small charges.
tolerance bands and reliability of plant be centered, adapting the tempering A complete belt furnace line installation
parameters. temperature. A common accepted value for for hardening and tempering is shown in
• Plant availability and productivity. Cm(k) or Cp(k) is > 1.67. The tolerance bands Fig. 1. It typically comprises in
• Lowest life cycle costs regarding. for the tensile strength prescribed for the sequence a container’s unloading
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