F.lli Temponi primarily performs thermal treatments on carpentry and cast iron pieces of all sizes The ovens and their control and management systems ensure the set thermal cycle accuracy and complete traceability, including remote, of the treated pieces.
At the Nave facilities we perform every type of cast iron treatment:
- Ferritisation annealing;
- Custom cycles (on specific instruction from the customer).
The control of each heat treatment cycle is primarily performed by sophisticated software in addition to traditional electronic monitoring systems that detect, record and archive all the cycles performed.
Cabin painting is performed by spraying with epoxy primers, either solvent-based and water-based, nitro-based, inorganic zinc and epoxy zinc-based primers. The colours are your choice. In terms of health and safety and looking after the environment (and with the help of qualified and reliable partners) we have introduced water-based epoxy products into our painting regimen, replacing the solvent-based products. This drastically reduces the rate of emissions of volatile organic compounds without altering their quality.
This quality is also guaranteed through the use of the most recent and sophisticated painting systems with head catalysis.
Verification of the applied thicknesses is carried out through special digital detectors.
The problem of thermally treating
The starting point for an in-depth analysis of the problem concerning the heat treatments of parts with rooms that are not suitably drilled comes from the fact that actually happened within our company. The heat treatment of a 172cm-long hollow metal cylinder with a diameter of 17.5 cm that was not properly equipped with vent holes by the manufacturer caused the pipe to burst, damaging both the load and structure of the oven.
Approximating the damage, which was fortunately only materials, the priority remained the investigation of the cause of the explosion in a heat treatment performed in air (without a controlled atmosphere and without the presence of fuel gas or explosive mixtures).
During annealing in the oven at 580°C, there was a break in the furnace at various locations with damage to the internal walls leading to the break-up of an area. In particular, serious damage occurred to a roller that had lost two plugs with an end that splayed like a daisy for more than half of its circumference.
Reflecting on the event
Based on the observations of the damage to the pipe and that of the oven shown in the photos, which in any case lead to an increase in pressure inside the roller present during annealing, the event may have been caused by:
1- Presence of explosive substances,
2- Explosion of gas mixture (gas, vapours, mists),
3- Burst upon reaching maximum pressure caused by the presence of liquids inside the tube,
4- Presence of reactive substances
A study performed by specialists in analysing the damage caused by explosive phenomena has shown that the most probable cause is that of point 3. Below is an excerpt from this study where the causes of the event are listed in detail.
To understand how the accident could have occurred with the damage to the annealing tube and the annealing furnace, we report the series of events that are likely to have occurred:
the tube contains liquid and/or powders casually inside
- the progressive increase of the oven temperature, up to about 580 ° C, causes the vaporisation of the liquid with the pressure value corresponding to the vapour pressure. Considering the quantity of liquid (3578 cm3) calculated from the traces inside the tube, it is possible to reach a pressure value of hundreds of bars (at least higher than 160 bar at 345.7°C). Considering the specific volume of saturated steam of 0, 00955 m3 / kg – calculated assuming the liquid consists of water, the tube volume equal to 34163 cm3 and the water weight is equal to 3578 g),
- aluminum and iron oxide powders inside the tube, that could have been located between the cap and flange, are triggered by the high temperature of the oven,
- the ignition of the powders, taking into account the reaction between the powders reaching a temperature of about 2700°C, generates a significant increase in the pressure that makes the caps jump,
- the increase in pressure between the cap and the flange reaches values that cause the caps and flanges to have an upside-down umbrella deformation due to the powders being located in this area,
- the pressure inside the pipe having exceeded that of resistance of the two plugs generates the expulsion of the same,
- the escape of the internal gaseous atmosphere determines the displacement of the tube by reaction. The tube moves in the opposite direction to the gash of the wall since the end towards the gash is completely free while the other end is not completely free due to the conformation of the plug. Despite having been expelled, it cannot make the area available for ventilation due to the proximity of another metal product in addition to having also attached the flange,
- the high value of the pressure in the area between the cap and flange a moment before the caps break down generates the daisy-like opening of the end which is also facilitated by the displacement of the tube which, by touching other pieces, has caused fractures,
- the cap of the torn end after ejection impacts on the neighbouring piece as the impressions clearly show,
- the other cap at the non-lacerated end impacts violently on the lower part of the bin-shaped product, which in turn produces a gash on the wall of the oven with the lower part opposite
- the escape of the gaseous atmosphere from the tube generates a reaction of the tube or a force that moves the bundle of rollers which ,in turn, moves all the neighbouing articles which, being adjacent to the walls of the oven, make it easy to break wall structure
- vigorous gas flow from the pipe completes the breaking and falling of the refractory panels with the relative resistances.
– Absolutely avoid closed chambers inside the parts which must undergo high temperature heat treatment.
– It is an obligation for all designers to be aware of the danger and to check that buildings with closed rooms are not delivered for heat treatment.
Bibliography and references:
 Technical report on damaged hollow cylinder – Ing. Nicola Mazzei