Union Carbide Corp. yesterday said second-quarter earnings were down 20 percent from a year ago, primarily because of lower sales, particularly in the petrochemicals and metals and carbon products industry segments.

Second-quarter net income was $101 million ($1.44 a share) compared with $126 million ($1.80) a year ago.

Sales for the 1985 second quarter were $2.26 billion versus $2.41 billion a year ago. The company had first-half sales of $4.43 billion versus $4.80 billion in 1984.

Second-quarter results included a one-time gain of $16 million (22 cents) from the sale of the corporation's U.S. welding and cutting systems business.

Earnings for the last six months totaled $172 million ($2.45), 26 percent below the $233 million ($3.31) total of a year ago.

*Monsanto Co. reported second-quarter net income of $124 million ($1.60), down from $145 million ($1.77) in the same period last year. Consolidated net sales for the second quarter were $1.63 billion, compared with sales of $1.80 billion in the second quarter of 1984.

For the first six months, consolidated net income was $211 million ($2.72), against $320 million ($3.90) for the comparable period last year.

Net sales were $3.25 billion for the first six months of 1985, and $3.53 billion for the same period last year.

*SmithKline Beckman Corp. reported net earnings of $121.4 million ($1.54) for the second quarter, a 1 percent increase from $120.2 million ($1.50) for the same period in 1984.

Henry Wendt, president and chief executive officer, said higher taxes and lower domestic sales of the ulcer medication Tagamet contributed to the slow growth in net income.

Wendt said Tagamet sales were down because of heavy trade stocking during the second quarter. However, he said foreign and U.S. sales of Tagamet grew during the first half.

*Warner Communications Inc. posted second-quarter net income of $28.6 million (41 cents), compared with a loss from continuing operations of $27.2 million (48 cents) a year ago.

The company said it had better results from its film and recorded music divisions, a gain from the sale of part of its stake in another company and reduced corporate expenses.

"These results reflect the success of the company's strategic decision to focus its attention on its core entertainment and communications businesses," said Steven J. Ross, chairman and chief executive.

Revenue slipped 1.7 percent to $467.4 million from $475.6 million a year ago.

The 1985 results include an after-tax gain of $8.2 million on the sale of 20 percent of Warner's stake in toymaker Hasbro Inc.

For the six months ended June 30, Warner reported net income of $50.0 million (72 cents), compared with a loss from continuing operations of $20.0 million (36 cents) a year ago. Its net loss in the first half of 1984 was $406.8 million. First-half revenue rose to $1.03 billion from $947 million a year ago.

*Armco Inc. announced second-quarter earnings of $18.5 million (24 cents) on sales of $994.2 million, up dramatically from the corresponding period in 1984, when the company suffered a net loss of $14.1 million (25 cents) on sales of $1.06 billion.

In the first six months of this year, Armco had a net loss of $4 million (12 cents) on sales of $1.91 billion, compared with net profit of $42.2 million (57 cents) on sales of $2.03 billion during the corresponding period in 1984.

The first half of last year included income of $170.4 million from the sale of Armco's coal operations and a provision of $105 million for future losses of its discontinued insurance businesses.

*The New York Times Co. said its second-quarter profit rose 13 percent on a 13 percent revenue gain over a year earlier. The company said net income for the three months totaled $31.3 million (78 cents), compared with $27.7 million (70 cents) in the same period a year ago.

Revenue came to $346.4 million versus $306.6 million for the year-earlier period.

For the first six months of the year, net income rose 22 percent to $63.5 million ($1.59) from $52.2 million ($1.32). Half-year revenue increased 12 percent to $674.3 million from $603.9 million a year ago.

*Crown Zellerbach reported net income and earnings per share were down significantly for the second quarter of 1985. Net income was $21.5 million (63 cents) on sales of $769 million for the quarter, compared with $32.5 million ($1.04) on sales of $772 million during the corresponding period of 1984.

*Republic Airlines, which had a record annual profit of $29.5 million in 1984, reported that it earned $120.1 million in this year's second quarter alone.

The record profit for the quarter ended June 30 included one-time gains of $62.2 million from pilot pension changes and $18 million from tax losses carried forward. But Republic also generated a second-quarter operating profit of $65.8 million on revenue of $455.6 million, compared with last year's operating profit of $43.7 million on revenue of $410.5 million.

Last year, Republic's second-quarter net earnings amounted to $29.5 million, including $11.7 million from tax losses carried forward.

*Ryder System Inc. said its second-quarter earnings from continuing operations rose to a record $35.9 million (75 cents a share) from $34.6 million (73 cents) last year, despite losses in its Distribution Systems Division. Revenue for the quarter reached $708.5 million, a 12 percent increase from revenue of $634.5 million a year earlier.

Ryder said revenue for the first half totaled $1.3 billion, compared with $1.2 billion in the first half of 1984. Earnings from continuing operations for the first half grew to $55.9 million, compared with $54 million.