Introduction: CR04 and Chain Bridge steels using both

Introduction:

 

QMAC Silos manufacture large
storage bins mainly for the agricultural industry. A key component of the silo
is the cone end ring that is attached to the end of the base cone. The cone end
rings are formed using Tata steel CR04 steel. QMAC silos are in the process of
changing material supplier to a company called Chain Bridge who can supply
equivalent steels from other European mills. The main issue with this is that
the alternative steel appears to fracture during the forming process where the
bolt holes are located in the blank. Preferably Qmac would still like to form
the part with the bolt holes in place. In this project we are to determine the
characteristic differences between the two types of steel or indeed if there are
any significant differences and analysis these.

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Aims and Objectives:

The aim for this project is to:

get an understanding of the material
characteristic’s for 2mm Tata CRO4 and Chain Bridge mild steel sheet material
under different constraintsDetermine potential solution that Qmac could use
to stop the deformation of the bolt holes during forming Objectives and deliverables:

Gain
information on the Qmac product and the end rings themselves through site
visit/CAD drawings
End ring
formation process what is involved to form end ring?
How are
the holes achieved on the sheet metal?
Information
on ASTM standards for tensile testing, Specimen standard for tensile
testing, shape and size of specimen
Review of
research for potential solutions and equations
Gain
knowledge in tensile testing and metal forming processes
Tensile
testing of Tata CR04 and Chain Bridge steels using both an-isotropic and
isotropic orientations
Analyse
the strains produced when forming the 62-degree end ring
Stress
concentration calculations for area around the holes
Knowledge
of using Roark’s Formulas for stress and strain calculations
Using
Roark’s formulas determine the stress and strain on the metals and produce
stress and strain relationship graphs to reflect findings
Feedback
information to Qmac with any recommended changes to the forming process
that would reduce the changes of deformation from happening around the
bolt holes on the end cones.

 

 

Literature review:

 

This is the review of material that
is relevant to my project as separated into appropriate sub headings below:

Material:

For the end ring manufacture Qmac
are using Cr04 / DC04 steel.

 

This steel has the European
standard EN 10130: 2006, It is specified as a

Cold rolled low carbon steel flat
product for cold forming. This steel has been designed especially to cope with
the stress and strains produced during cold forming processes. (Tata Steel , 2017 )

 

Technical data sheets will be
soured for the two different steels.

 

Cold working:

 

Cold working, is a metal forming
process that is carried out at room temperature. In cold working, plastic
deformation of process causes strain hardening. The force required to shape a
part is much larger than if the part was being hot worked. At cold working
temperatures, the ductility of a metal is limited, and only a certain amount of
shape can be created on the material before it starts to deform. This is a
crucial point when looking at the ring end as because it has pre drilled holes
added more fracture areas that are more susceptible to a high stress
deformation area. The main problem with the cold working process is that the
metal has a poor ductility and is prone to stress concentrating and tearing
around the forming area. The advantage of cold forming is that the ring end
will be stronger and harder due to strain hardening that happens during the
process. Cold forming causes directional grain orientation, which can be
controlled to produce desired directional strength properties for the finished
part this directly relates to the part being analysed.

(The library of manufacturing , 2017 )

 

Process for forming end ring:

The process of making the end rings
is as follows,

 

2mm
DC04 Sheet is punched into blanks with holes The
blank is rolled into a cylindrical shape The
ends of the rolled cylinder are MIG welded to form one pieceThis
cylindrical blank is then formed cold working into the conical shape using a
hydraulic press with a die.  

Factors affecting material after forming
process:

Spring Back:

According to (Gate, 2015) Spring back is
considered as one of the major problems in press forming which occurs during
the unloading of the formed part from the forming tools. For the accurate
prediction of spring back, unloading behaviors of cold rolled steel DC04 for
the process should be investigated and a strategy for the reduction of spring
back based on variable blank holder force considered. This is related to Qmac
end rings as there could be potential in the forming process for spring back
and this has a potential to deform the material and could lead to stress
concentrations around where the holes are deforming.

 

(Bauer, 2016) Suggests that in the
rolling process distribution of residual stresses in the blank and the
direction of the stress tensors allows for a definition of the blank in the
rolling process even before the pressing process. He suggests that the
temperature and friction of the rolling process can induce additional stresses
on the material. This could potentially increase stress concentrations in the
rolled blank tubes formed by Qmac before they press flange the formed pipe
allowing for deformations to occur at a faster and more aggressive rate.

 

Variability in process:

 

There could be variability in the
process due to wear on tooling which could cause induced stress areas on the
sheet when in the forming process. Over time tooling will degrade and become
out of specification this should be checked on a regular basis to ensure all
forms are produced within spec to avoid any addition stresses and material
thinning and thickening which could cause potential failures.

 

Differing hydraulic pressures could
be used to form the component this could cause an excessive compression force
on the formed part and could cause it to give way (deform) around vulnerable
areas such as the bolt holes on this part.

(The library of manufacturing , 2017)

 

Thickness variations of incoming material, the
thickness of the DC04 sheets could vary from batch to batch and this could
cause an effect on the overall stress and stain when cold forming the ring end.
There is also potential that thin areas could have increased stress concentration
and in some cases thin wall and sheer/deform due to this factor. Depending on
the thickness it could have a different effect on the forming process and could
cause molecular variations on the structure of the finished part.

(Tekkaya, 2012)

 

Poor punch tool quality

 

 

 

 

 

 

Effect
of temperature during forming process:

Properties of a metal change with
an increase in temperature. Therefore, the metal will react differently to the
operation if it is performed under different temperatures and the end ring have
differing characteristics because of this process.  

 

Friction between the tooling and metal being formed:

Metal forming processes are normally a form applied with high
pressures between two contacting surfaces (top and bottom mould form). A
certain amount of friction will be necessary for the forming process, but
excessive friction is always avoided as best. Friction increases the amount of
force required to perform the forming operation, causes wear on tooling, and
can affect metal flow causing failure fatigue and creating defects in the formed
piece. Lubricants can usually help to reduce the friction factor of the
process. Lubrication is applied to contacting surfaces to reduce friction
forces. Lubricants used in forming processes are different depending on the
type of metal forming process, the temperature at which the operation occurs,
and the type of metal that is being formed.

 

Formability and Factors affecting formability:

 

What is formability?

Formability is the ability of sheet
metal to undergo shape change without failure by necking or tearing. It is the
measure of the value of deformation a material can withstand before thinning or
a fracture occurs. This is a measure that is essential when manufacturing or
analysing materials as materials can vary in each batch produced so should be
reviewed on a regular basis.

(Industrial
Metallurgists, 2017 )

Factors affecting formability;

 

Micro cracking:

“An anticipated failure mode is
captured where formability is limited by severe macro-cracking preceded by
localization with void sheeting” (Soyarslan, 2011)

This is where a micro crack appears
and can become large enough to create a void in the metal. At this void there
will be the lightly hood of a failure as the key properties of the metal have
exceeded their elastic limit and will yield when forming operation is
attempted.

 

Chemical composition:

Where the chemical form of the DCO4
metal could differ to that of which is in its specification this could cause
irregular deforming in areas where the composition is different and could cause
areas of high stress in the forming process of the end ring. Many metal alloys
have specific compositions to achieve a desired strength, microstructure, or
corrosion resistance, Composition Analysis can be used to determine if the CRO4
has any impurities or other unusual elements present that could cause
failures.  Chemical composition is tested
by the following methods, Metallography and Energy Dispersive X-Ray
Spectrometry (EDS), Inductively Coupled Plasma (ICP), X-Ray Photoelectron
Spectroscopy (XPS) all these methods would involve sample material being sent
away for analysis. This has shown that many metals have had differing chemical compositions
than what was on their specifications.

 

(Polymer Solutions, 2017)

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