Forging, whilst simple in concept, can present a multitude of technical challenges which can determine component geometry, process conditions and tooling criteria.
Working with our customers, Uddeholm have developed a family of steels addressing four of the main tool failures that account for the vast majority of all tool failures associated with a forging process: hot wear, heat checking, plastic deformation and premature cracking.
What's the problem?
Simply put, forging is the process whereby a heated up billet is pressed between a die set to an almost finished product. Large numbers of solid metal parts are produced in aluminium alloys, copper alloys, steel or super-alloys where irregular shapes need to be combined with good mechanical properties. The main methods of drop forging are hammer forging and press forging.
Forging is an aggressive process requiring a good deal of shock resistance and high temperature performance. The material being shaped also has an effect on the performance of the tool; for materials such as Titanium, where billet temperatures can reach up to 1100°C, the elevated temperature compounds the demand placed on the tool.
The issues outlined below are not isolated, but can work together to reduce the life of the tool. Forging dies are often measured in the hundreds of components output as opposed to plastic injection moulding, and indeed high pressure die casting where the output is often measured in the hundreds of thousands and even millions of components.
Hot Wear
Hot wear and heat checking present as surface cracking or deterioration of the die surface. Repetitive cycles of thermal change expand and contract the surface, forming micro-cracking, which results in issues with component quality either visually or dimensionally. This will also cause stress raising in localised areas ultimately leading to a gross cracking failure. Experimenting with hardness can help with hot wear to a degree but a chemical composition of the steel is the best way to maximise high temperature performance through the addition of elements such as Molybdenum & Vanadium.
Plastic Deformation
Plastic deformation becomes an issue when the surface of the tool can no longer resist external pressures. There are multiple reasons why this can happen, but the primary cause is the heat in the process causing the surface to soften below the desired hardness. At this point, the massive loads typically involved in forging overwhelm the steel, causing it to behave more like plastic. Compressive strength is a direct function of hardness, so to resolve the issue we need to maximise the hardness and tempering back resistance of the steel. There is often a resistance to increasing the hardness as it does have the effect of reducing the maximum toughness so can be more prone to cracking, contact us here to discuss with one of our Engineers who can advise on potential improvements. It is also critical to ensure that any re-cuts of the die surface have removed the 'Tempered-Back' steel to ensure the hardness is at maximum levels.
Gross Cracking
A combination of all the issues already mentioned can lead to gross cracking; the most disruptive, dangerous and perhaps most common failure of forging dies. This failure is the most critical to resolve and must be looked at before addressing the other failure modes. If your dies are cracking, you are likely not reaching wear or plastic deformation issues, which is where we aim to get too. Tool wear may be inevitable, tool failure is not! The toughness and ductility of the steel are paramount to resolving cracking issues, followed closely by the consistency. H13 for example is a commonly used forging die steel which has a relatively broad spectrum as far as chemical composition is concerned; one batch may have a Chromium content of 4.75% and another 5.50% both would be acceptable H13 but have different hardening characteristics and performance. This could be the difference between two identical dies failing. All Uddeholm steels have strict controls on the chemical composition to guarantee performance.
How do I fix it?
Speak to us. Our Engineers can help identify issues and potential areas for improvement.
Shown below is a comparative chart of the Uddeholm hot work grades which are suitable to forging. Commonly used grades such as H11, H13, 1.2714, 1.2367 etc. will give our Engineers a starting point on how to begin resolving the common issues identified in the chart, then using our experience with a range of forging applications, we can often offer novel solutions to niche demands. This is a good starting point to identify potential grades which will help improve issues you may be having.
Notch toughness indicates the ability of the die material to resist crack development from such defects. All products in the Uddeholm tool steel programme for the forging industry are characterised by the highest levels of toughness and ductility in all directions in the bar or block. Hence, the forger can rest assured that the resistance to gross cracking is the highest possible in dies made from Uddeholm die steel. Proper die preheating will considerably reduce the risk for catastrophic failure via cracking.
The better the steel retains its hardness as the temperature or the time increases, the better its temper resistance. Temper resistance can be assessed from the tempering curve for a hardened tool steel. In this, the hardness at room temperature is plotted against tempering temperature for given tempering time. Another method of presenting temper resistance data is to plot room temperature hardness against time at a given tempering temperature.
Uddeholm tool steel for forging dies are produced to the highest possible quality standards, especially with regard to freedom from nonmetallic inclusions. This imparts a degree of fatigue resistance which is adequate for even the most demanding applications where forging dies are subjected to cyclic loading with high maximum loads.
Uddeholm have been solving tooling challenges for over 350 years. All of our steel is made to the highest of standards and is always manufactured at our mill in Sweden from recycled steel and is fully traceable. Our UK office and laboratory can support with all your enquiries. Visit our website here.
Written by Ben Broderick, Technical Sales Engineer at UDDEHOLM UK
