Hardness testing as a method to identify the highest-temperature combustion zone in transport fires

. This article presents some results on the selection of the necessary micro-hardness tester for the purposes of research. In accordance with the objectives, namely the scientifically based choice of a hardness meter for the purposes of fire-technical examination and evaluation of its capabilities, aimed primarily at the possibility of identifying the most high-temperature combustion zone in fires in transport. The selection of a device for measuring microhardness was carried out in accordance with current methods for measuring microhardness, first of all for determination of microhardness for products based on metals and their alloys, as well as materials found in vehicles. The paper describes the main types of hardness testers and their applications. On the basis of the analysis programmable electronic small-sized hardness tester TEMP-4 was chosen. This device met all the requirements on the decision of set tasks of research connected with express researches both laboratory and industrial conditions, and the field at the decision of tasks of fire-technical examination directly on a place of ignition of the transport unit. Experimental results of nondestructive express measuring of various metal samples are described. Metal fasteners and supporting constructions are chosen as samples for research. The thermal effect on the test specimens was carried out in a thermostat chamber allowing for an impact heating rate. The results, testifying about change of microhardness of metal products as a result of influence of a high-temperature field are received.


Introduction
At the present time fires on transport arising for various reasons, including because of various kinds of accidents except for the big material and ecological damage do irreparable damage to the population of Russia [1,2,3]. Besides, the available statistical data does not allow us to cover in full the whole list of transport means: sea, air, railway, cargo, including the specialized transport serving quarries and open-cast mines of mineral raw materials complex of Russia, and also automobile transport [1,3]. The statistical data given in table 1, provided by the Ministry of Transport of Russia (Rostransnadzor), only confirms the necessity of carrying out this kind of researches. They are aimed both at developing measures to reduce accidents and at establishing the true causes and locations of hotbeds of combustion, as well as the dynamics of flame spread through the main communications and structures of vehicles operating on different types of fuel. One of the methods for determining the location of combustion and the dynamics of propagation of the combustion front as part of complex techniques, as shown by our research, can be a method of non-destructive express control of hardness of the metal included in the structural composition of vehicles [3,4,5]. Depending on the instrumentation used hardness meters can measure the hardness of a number of other materials included in the composition of vehicles besides metals [1,6,7].

Materials and methods
In carrying out the research work, some of the results of which are discussed in this article, micro-hardness methods were chosen as methodological and logistical support, and several types of micro-hardness meters were used in the instrumentation [4,8,9].
The most common micro-hardness testers, depending on their operating principle, can be ultrasonic (UHT), dynamic (DHT) or universal micro-hardness tester [3,9,10]. The basis of operation UHT is the contact ultrasonic impedance method (UCI method). In contrast to UHT, DHT works according to the Leeb method. It takes into account the relationship between the rebound velocity and the falling velocity of a carbide-tipped indenter and the hardness of the material being tested [11,12,13].
Whereas, in the early days of metal hardness research, Rockwell and Super Rockwell, Brinell, Shore and Vickers hardness testers were used, each with its own hardness scale. The micro-hardness scales were named after their inventors, e.g. the Vickers hardness tester had a Vickers scale of HV.
With the development of technology and scientific knowledge, i.e. the development and improvement of instruments in modern types of hardness testers, it is now possible to interpret the data obtained from these scales [12,13,14].
The metal hardness results presented in this article were obtained by using a microhardness tester TEMP-4 (electronic compact portable programmable hardness tester). The choice of this type of micro-hardness tester is justified by its versatility and the possibility, if necessary by programming the built-in scales, to extend the capabilities of the device and examine not only a wider range of metals, including cast iron, but also some non-metals, such as rubber. Besides, this type of device allows it to be used by various specialized services (for example, during express fire-technical data collection directly on the fire place), i.e. in field conditions, as well as in laboratory and production conditions at enterprises of wide technical profile. The objects to be measured can be of different shapes and thicknesses [12,14,15].
This type of instrument was used in various modes of operation during the research work. The main purpose was to carry out express measurements of the micro-hardness of steels in a non-destructive way. In this article the determination of the micro-hardness of steels should also be understood to mean that the authors measured not only "pure" metals, but also their alloys and their welds and joints.
The Brinell scale (HB) and other types of scales, namely Vickers (HV), Shore (HSD) and Rockwell (HRC), are primarily used for micro-hardness determination. In addition, it can be used to determine the ultimate tensile strength of steels, Rm. The TEMP-4 thus allows you to obtain hardness results directly in hardness numbers (HV, HRC, HSD and HV).
One of the main aims of this research work, some of the results of which are summarised in this article, was to determine the capability of the device to operate under various conditions when carrying out fire technical examinations.

Results
The test specimens (nails and screws) were placed one after the other in a furnace heated to a preset temperature and kept there for 15 minutes each, then cooled down naturally.
The temperature is monitored and controlled by an electronic regulator. The maximum annealing temperature was 1200°С.
The samples were then heated over a temperature range of 100 to 1000°C. For each new sample, the temperature was increased by 100°C, with an oven dwell time of 15 minutes. After the heat treatment the samples were cooled down naturally.
A visual analysis of the surface changes of the samples after the heat treatment is shown in Figures 1 and 2.
The visual analysis of the examined samples showed that it is possible to distinguish clearly between the changes occurring to the samples depending on the exposure temperature. Metal corners when heated to a certain temperature and allowed to stand for 15 minutes: 100÷200 °C -There is no visible change. 300 °C -The steel corner has turned yellow-brown. 400 °C -The corner has taken on a dark brown colour. 500 °C -The grey is now a shade darker.   Table 2 and, for better visualisation, additionally represented by the graphs for the metal angles in Fig. 3 and for screws in Fig. 4.  TransSiberia 2023  600   510  507  484  481  476  490  491  477  481  484  488  12   700  486  490  493  502  481  478  480  484  480  482  486  7   800  480  483  487  503  493  494  477  478  480  485    As a result of the research work, some of the results presented above have shown that the dependencies shown in Figures 3 and 4 are true for all metal products present in trucks and cars.

Conclusion
Several conclusions can be drawn from the experiments carried out and the experimental data obtained.
As a field device, the development of a small portable programmable TEMP-4 hardness tester was scientifically justified. This device proved itself well, especially in experimental studies of steel products, cold-deformed products subjected to thermal effects, similar to the rate of temperature increase in a hydrocarbon fire.
According to the obtained dependencies on the change in the micro-hardness of the samples on the exposure temperature, it was found that the data obtained in the interval from 600 till 1000 0С coincide well with the results of changes in the magnetic characteristics of similar materials after exposure to high temperatures. In this range, the recrystallisation process results in a decrease in hardness of the investigated metallic samples and materials.
We consider that application of a micro-hardness metre of the given type or not inferior on tactical and technical characteristics, type of execution and functional opportunities, undoubtedly, will expand a set of devices applied for the purposes of fire-technical expert examination. This applies to technical measuring instruments that are used directly on site, i.e., in the field, and serve both to locate the initial origin of the fire, i.e., to help establish the location of the fire, and indirectly to determine the cause of the fire. Thus, measuring the microhardness of large metal parts (steel bodies) of burnt-out vehicles will provide the necessary data to assess the rate and time of heating. In addition, it will identify (determine/identify) the areas subjected to the highest temperature impact, which is particularly relevant for load-bearing steel structures.
The results obtained by the authors do not contradict the available data and are in good agreement with the results of other researchers [1,8,12,14]. The authors suggest that this kind of research and its results are of some interest to specialists in the field of fire expertise, so the work in this direction will continue.