Spark plug and method of producing spark plug

A spark plug comprising:

• a central electrode;
• an insulator surrounding the central electrode radially;
• a metallic shell surrounding the insulator radially;
• and
• a ground electrode having a first end connected to the metallic shell and a second end defining a side face, the ground electrode being so bent that the side face of the second end faces the central electrode, at least a surface layer of the ground electrode containing:
• a nickel in a range from 58% to 71% by weight,
• a chromium in a range from 21% to 25% by weight,
• an iron in a range from 7% to 20% by weight, and
• an aluminum in a range from 1% to 2% by weight,
• in which the at least the surface layer of the ground electrode has a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244, a load 9.8 N being applied to the ground electrode in the Vickers hardness test.

A ground electrode of a spark plug, the spark plug having a central electrode;

• an insulator surrounding the central electrode radially;
• and a metallic shell surrounding the insulator radially;
• the ground electrode having a first end connected to the metallic shell and a second end defining a side face, the ground electrode being so bent that the side face of the second end faces the central electrode, at least a surface layer of the ground electrode comprising:
• a nickel in a range from 58% to 71% by weight,
• a chromium in a range from 21% to 25% by weight,
• an iron in a range from 7% to 20% by weight, and
• an aluminum in a range from 1% to 2% by weight;
• in which the at least the surface layer of the ground electrode has a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244, a load 9.8 N being applied to the ground electrode in the Vickers hardness test.

A method of producing a spark plug having a central electrode;

• an insulator surrounding the central electrode radially;
• a metallic shell surrounding the insulator radially;
• and a ground electrode having a first end connected to the metallic shell and a second end defining a side face, the ground electrode being so bent that the side face of the second end faces the central electrode;
• the method comprising the following sequential operations of:
• preparing the ground electrode composed of an alloy material containing:
• a nickel in a range from 58% to 71% by weight,
• a chromium in a range from 21% to 25% by weight,
• an iron in a range from 7% to 20% by weight, and
• an aluminum in a range from 1% to 2% by weight;
• annealing the alloy material of the ground electrode at an annealing temperature not lower than 800° C., so as to allow the alloy material of the ground electrode to have a Vickers hardness in a range from Hv 140 to Hv 220 measured through a Vickers hardness test specified in Japanese Industrial Standard Z2244, a load 9.8 N being applied to the ground electrode in the Vickers hardness test;
• welding the ground electrode to the metallic shell;
• and
• bending the ground electrode in such a manner as to allow the side face of the second end of the ground electrode to face the central electrode.