научная статья по теме CHARACTERIZATION OF MICROSTRUCTURE OF H13 TOOL STEEL USING ULTRASONIC MEASUREMENTS Общие и комплексные проблемы технических и прикладных наук и отраслей народного хозяйства

Текст научной статьи на тему «CHARACTERIZATION OF MICROSTRUCTURE OF H13 TOOL STEEL USING ULTRASONIC MEASUREMENTS»

УДК 620.179.16

CHARACTERIZATION OF MICROSTRUCTURE OF H13 TOOL STEEL USING ULTRASONIC MEASUREMENTS

Iwen Mutlu1, Enver Oktay1, Sinasi Ekinci2 1 Istanbul University, Department of Metallurgical and Materials Engineering, Istanbul, Turkey E-mail: imutlu@istanbul.edu.tr 2 Industrial Application Department, Cekmece Nuclear Research and Training Center, Istanbul, Turkey

Abstract. This study presents nondestructive characterization of microstructure of AISI H13 hot work tool steel. Heat treatments were carried out in order to obtain different microstructural phases in the tool steel specimens. The microstructural phases were characterized by metallographic examinations and hardness measurements. Velocities of ultrasonic longitudinal and transverse waves were measured by means of pulse-echo method using contact type normal beam probes. Ultrasonic apparent attenuation also determined in the steel specimens having different microstructural phases. A lower value of ultrasonic velocity was observed for the martensite compared to the other microstructures, while the opposite was observed in ultrasonic attenuation. Results show that the use of ultrasonic measurements in order to correlate them with the microstructures is fast and reliable, permitting nondestructive characterization of microstructure in steels.

Keywords: Ultrasonic, Heat treatment, Microstructure, Grain size, H13 tool steel.

ОПРЕДЕЛЕНИЕ ХАРАКТЕРИСТИК МИКРОСТРУКТУРЫ ИНСТРУМЕНТАЛЬНОЙ СТАЛИ Н13 С ИСПОЛЬЗОВАНИЕМ УЛЬТРАЗВУКОВЫХ ИЗМЕРЕНИЙ

Илвен Мутлу1, Энвер Октай1, Синаси Экинси2 1 Стамбульский университет, факультет металлургии и механики,

Стамбул, Турция 2 Департамент промышленных приложений, Ядерный исследовательский и испытательный центр, Стамбул, Турция

Исследовали способ определения характеристик микроструктуры закаленной инструментальной стали Н13 на основе неразрушающих у. з. измерений. Проводили термическую обработку с целью получения различных микроструктурных фаз, которые определяли с помощью металлографических исследований и измерения твердости.

Скорости у. з. продольных и поперечных волн были измерены эхометодом с использованием пьезопреобразователей. Дополнительно определяли ослабление у. з. волн, которое также является характеристикой микроструктуры стали Н13. Получены меньшее значение скорости ультразвука и повышенное затухание в мартенситных микроструктурах по сравнению с другими типами микроструктур. Результаты исследований показывают, что акустические параметры коррелируют с микроструктурным состоянием стали. На основе у. з. измерений есть возможность оценить микроструктурное состояние, что дает быстрый и надежный способ определения микроструктуры стали неразрушающим методом.

Ключевые слова: ультразвук, тепловые испытания, микроструктура, размер зерна, инструментальная сталь Н13.

1. INTRODUCTION

Tool steels are commonly used to make tools, moulds and dies for cutting, forming or shaping a material into a part for component because of their high strength and wear resistance. There are various grades of tool steels, such as cold work tool steels, hot work tool steels, and high speed tool steels. Hot work tool steels are commonly used at temperatures higher than 200 °C. They resist softening up to 550 °C temperature [1—2].

AISI H13 steel alloy is a Cr—Mo—V based hot work tool steel which is characterized by high resistance to thermal shock and thermal fatigue, good high temperature strength, exellent toughness, ductility, machinability, air hardenability, resistance to abrasion and high hardness. Typical applications of AISI H13 hot work tool steel are hot punches and dies for blanking, bending, swaging and forging, hot extrusion dies for aluminium, cores, ejector pins, inserts and nozzles for aluminium, tin and lead die casting. Hardness and microstructure of the tool steels for specific applications should be in a defined range. Hardness of the hot work tools steels for die casting should be 52—54 HRC, steel for extrusion should be 40—48 HRC, steel for hot pres should be 44—52 HRC, and steel for wear resistant parts should be 50—52 HRC [1—2].

Steel is an attractive material due to possibility of making changes in its mechanical properties by changing the microstructure. Steel properties can change from charge to charge or after heat treatment. In addition, properties of steels can change during service. Therefore, it is necessary to develop a test system which implements fast nondestructive way for determination of mechanical properties by characterizing the microstructure.

Ultrasonic measurements can be used to characterise the microstructure and to estimate grain size in materials [3—12]. It is advantageous to use a nondestructive method for evaluate microstructure and measurement of grain size of a material since the optical method is time consuming and requires cutting specimens from component. Additionally optical method allows only surface or limited volume measurements to be made and gives the results only at these selected locations.

The velocity of the ultrasonic wave in a homogeneous medium is directly related to Young's modulus and density of the material; thus changes in either elasticity or density will effect pulse transit time through a specimen. Additionally, ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The microstructural phases also govern the mechanical properties. Hence, a possibility exists for assessment of the microstructural phases and mechanical properties through the ultrasonic measurements [3—10]. The velocity of ultrasonic wave and energy losses by interactions with the microstructure are the key factors in ultrasonic characterization. These parameters can be used to determine the Young's modulus, microstructure, texture and mechanical properties [9].

Ultrasonic grain size determination of materials has been studied by several nondestructive techniques which are dependent on ultrasonic parameters such as ultrasonic attenuation, ultrasonic backscattering, and ultrasonic velocity. By the ultrasonic attenuation method, the amplitudes of backwall echoes are used to determine attenuation. In the ultrasonic backscattering method, grain noise signals used for the grain size determination. The ultrasonic velocity technique depends on the velocity-grain size relation. Ultrasonic velocity is dependent on sample structure and affected by scattering of ultrasonic waves in the sample.

There are some studies correlating ultrasonic properties with microstructural phases in stainless and carbon steels [3, 6—9], reporting that grain size, microstructural phases and their morphology affect the parameters of ultrasonic waves. The propagation rate of ultrasonic waves is controlled by Young's modulus and density. It is influenced by the microstructure through changes in the Young's modulus of the individual grains, orientation of the grains by texture, and second phases [9]. Measurements in Jominy-specimens have shown variations of ultrasonic attenuation and velocity due to the structure heterogeneity, induced by phases distributed differently inside specimen [12].

The aim of this study is to investigate the microstructural phase-ultrasonic parameter relationship in AISI H13 hot work tool steel specimens. Considered

microstructural phases were martensite, bainite, pearlite-bainite and ferrite. For this purpose, the specimens having known microstructures were obtained by heat treatments and characterised by ultrasonic measurements. Grain size-ultrasonic velocity relationship was also studied. Although, there are some studies correlating ultrasonic properties with microstructural phases in stainless and carbon steels, there is no detailed study on non destructive characterisation of microstructure and grain size of tool steels in literature. This study also presents experimental data about non-destructive quality control to die manufacturers, and heat treatment industry.

2. EXPERIMENTAL

AISI H13 grade hot work tool steel (Uddeholm Tooling AB, Sweden) with chemical composition consisted of 0.41 % C, 5.26 % Cr, 1.46 % Mo, 0.95 % V, 1.07 % Si, 0.45 % Mn, 0.13 % Cu, 0.004 % S, 0.008 % P and balance Fe was used in this study. The steel was supplied in annealed condition. Five AISI H13 hot work tool steel specimens of 25x50x75 mm in size were cut from a bar. Initial ultrasonic and metallographic inspections on the bars did not show any porosity, inclusion, and other imperfections that may affect the experimental results.

The surfaces of the specimens were ground to achieve sufficient flatness and parallelism. AISI H13 tool steel specimens were heat treated at different temperatures for different holding time in order to obtain the specimens with varied microstructural phases. One specimen was studied in as-received (annealed) condition, which consists of ferrite matrix phase with carbides. The heat treatment process consisted of pre-heating at the temperatures of 450, 650 and 850 °C for 1 hour and then austenitizing at 1030 °C for 30 minutes, following by oil quenching to 500 °C and then air cooling to 50—70 °C. The quenched steel specimens were then tempered at 570 °C for 90 minutes as soon as reach to 50—70 °C. The heat treatment process of AISI H13 steel specimens was performed in a BaCl2 based neutral salt bath. Preheating serves to minimize thermal shock, equalize temperature, and minimize amount of time required at high temperature stage. All specimens were austenitized under same conditions to produce same grain size and grain size distributions. Isothermal heat treatments have been applied to the steel specimens to obtain bainite and pearlite-bainite phases. The steel specimens were cooled to 400 °C and holded for 3 hours to obtain pearlite-bainite microstructure. Pearlite was obtained by furnace cooling after austenitizing.

In addition, non-destructive grain size determination by ultrasonic velocity measurements has been studied. Four AISI H13 hot wo

Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.

Показать целиком